IMO, the executive summary answer is to install a 20 amp power inlet on the side of your camper and plug into the 20 amp pedestal recept. This should include a 20 amp extension cord and 20 amp romex inside to the AC unit.
The full text wordy (my apologies) explanation of why is here:
The current edition of the NEC only requires an RV park to have 20% of the pedestals to be 50 amps. In 2002, the NEC only required 5% @ 50 amps. These are the minimum requirements and installing more than 20% gets very expensive due to the cost of all the extra copper, conduit, etc. so unless you intend to use only high end RV parks that charge a premium rate to cover the higher installation cost, you are going to only find a max. of 20% of the RV park at 50 amps. And this is at newer RV parks.
In the height of the RV-ing season, the competition for any 50 amp pedestals is extreme and you will have trouble getting one. You can spend a LOT of $$ on upgrading to 50 amps and then not find a 50 amp pedestal at a CG/RV park. I have seen many high end MHs with a 50/30 adapter at the pedestal and then a separate extension cord running out a window to the 20 amp receptacle... In addition to MHs, nowadays there's lots of FWs & TTs with 50 amps as well. RV manufacturers craftily do not tell folks about the scarcity of 50 amp recepta. in RV parks.
"Modern" pedestals are loop fed with heavy ampacity (current) conductors, like say 200 amps (depends on number of pedestals on a run). A pedestal with a 30 amp and a 20 amp receptacle on it will be supplied off a large amapacity feeder cable like the 200 amps. Unless every other RV on that run are running 2 ACs and have their 30 or 50 amp services running full tilt, you should normally not have an issue.
Older RV parks (like in 60s & 70s) typically used a "star" network with a large feeder cable run to a remote panelboard in a grouping of RV sites, and then each site has a 30A sub-feeder with a code demand factor of 3600 watts (30 amps). Using a 30 + 20 amp recept. simultaneously at one of these pedestals will likely be a problem.
The NEC also has minimum conductor (wire) sizing depending on the number of pedestals connected on a feeder. For ex., 7 - 9 sites on a feeder run has a demand factor of 55%. The 2014 & 2002 codes are the same on this. If everyone ran full tilt (30 amps) on a feeder with 7 sites, the "main" breaker (probably 200 amps in this case) supplying this feeder back at the RV park main service location, would trip.
All of the above assumes the voltage is at or very close to 120 volts. As voltage drops, and it usually does in RV parks in the summer due to all the AC units running, your current will go up along with everyone else's. Impossible to predict what will happen and depends on circumstances at each specific RV park, the number of users and the park's wiring design. When some say that they can run 2 ACs on a 30 amp service, that will be close to 120 volts. Using an autoformer will help, but not if a lot of other RV-ers using pedestals on the same feeder are using one.
If you run the 2nd AC off the 20 amp recept. in a pedestal, if you roll into an RV park and have a choice of 30 or 50 amp pedestals, go for the 50 amps and use a 50/30 adapter. This can reduce the amount of voltage as the feeder cable is heavier gauge to accommodate the higher 50 amp current.
An AC unit draws up to 6 times its running current when starting up. A 30 amp breaker may trip sometimes on startup. If you have other things turned on or plugged in, try turning them off. If at a marginal point, this may enable you to run the 2nd AC. Damage to AC units will occur in the range of 102 - 105 volts. If voltage is getting low like say 108 or so, forget about trying to run the 2nd AC. Running a 2nd AC off the 20 amp pedestal recept. will NOT help if the CG voltage gets really low.
The last thing to be aware of is that a 30 amp breaker does not trip the instant it reaches 30 amps. These breakers have a standard inverse time-current characteristic as shown in the graph below. More readable image here. A breaker will continue to operate longer under a small overload and will trip faster under a higher overload. For example, the graph shows that at 45 amps, the breaker will run for somewhere between 100 & 400 seconds before it trips or for approx. 200 - 1,000 seconds at 38 amps. What this means is that you can run a MW or a toaster for a minute or two and be fine. However - depends on the age of a CG pedestal breaker and it's use/abuse.
If you go for the 20 amp power inlet, you might consider installing a 20 amp SPDT switch and a dedicated 15/20A T-slot recept. inside somewhere. This way, in cooler weather when not using the AC units, you have the ability to plug in a high wattage appliance and not worry about tripping a 30 amp breaker.
Caveat: Staying at a "modern" CG/RV park (done to more recent NEC code) and using a separate 20 amp power inlet to feed an AC unit is still not a guarantee that you will have no issues with running 2 ACs, but it will greatly improve your chances of having success. Choosing a site closest to the park's main electrical service (or near a pad-mounted transformer) can also help (less voltage drop). Also a good idea to check the pedestal voltage beforehand - if it's already marginal, it's only going to get worse when you plug in. Having a permanent voltage monitor inside is a good idea. I have an LED readout in our living area and can see it regularly go up and down like a yo-yo.
The full text wordy (my apologies) explanation of why is here:
The current edition of the NEC only requires an RV park to have 20% of the pedestals to be 50 amps. In 2002, the NEC only required 5% @ 50 amps. These are the minimum requirements and installing more than 20% gets very expensive due to the cost of all the extra copper, conduit, etc. so unless you intend to use only high end RV parks that charge a premium rate to cover the higher installation cost, you are going to only find a max. of 20% of the RV park at 50 amps. And this is at newer RV parks.
In the height of the RV-ing season, the competition for any 50 amp pedestals is extreme and you will have trouble getting one. You can spend a LOT of $$ on upgrading to 50 amps and then not find a 50 amp pedestal at a CG/RV park. I have seen many high end MHs with a 50/30 adapter at the pedestal and then a separate extension cord running out a window to the 20 amp receptacle... In addition to MHs, nowadays there's lots of FWs & TTs with 50 amps as well. RV manufacturers craftily do not tell folks about the scarcity of 50 amp recepta. in RV parks.
"Modern" pedestals are loop fed with heavy ampacity (current) conductors, like say 200 amps (depends on number of pedestals on a run). A pedestal with a 30 amp and a 20 amp receptacle on it will be supplied off a large amapacity feeder cable like the 200 amps. Unless every other RV on that run are running 2 ACs and have their 30 or 50 amp services running full tilt, you should normally not have an issue.
Older RV parks (like in 60s & 70s) typically used a "star" network with a large feeder cable run to a remote panelboard in a grouping of RV sites, and then each site has a 30A sub-feeder with a code demand factor of 3600 watts (30 amps). Using a 30 + 20 amp recept. simultaneously at one of these pedestals will likely be a problem.
The NEC also has minimum conductor (wire) sizing depending on the number of pedestals connected on a feeder. For ex., 7 - 9 sites on a feeder run has a demand factor of 55%. The 2014 & 2002 codes are the same on this. If everyone ran full tilt (30 amps) on a feeder with 7 sites, the "main" breaker (probably 200 amps in this case) supplying this feeder back at the RV park main service location, would trip.
All of the above assumes the voltage is at or very close to 120 volts. As voltage drops, and it usually does in RV parks in the summer due to all the AC units running, your current will go up along with everyone else's. Impossible to predict what will happen and depends on circumstances at each specific RV park, the number of users and the park's wiring design. When some say that they can run 2 ACs on a 30 amp service, that will be close to 120 volts. Using an autoformer will help, but not if a lot of other RV-ers using pedestals on the same feeder are using one.
If you run the 2nd AC off the 20 amp recept. in a pedestal, if you roll into an RV park and have a choice of 30 or 50 amp pedestals, go for the 50 amps and use a 50/30 adapter. This can reduce the amount of voltage as the feeder cable is heavier gauge to accommodate the higher 50 amp current.
An AC unit draws up to 6 times its running current when starting up. A 30 amp breaker may trip sometimes on startup. If you have other things turned on or plugged in, try turning them off. If at a marginal point, this may enable you to run the 2nd AC. Damage to AC units will occur in the range of 102 - 105 volts. If voltage is getting low like say 108 or so, forget about trying to run the 2nd AC. Running a 2nd AC off the 20 amp pedestal recept. will NOT help if the CG voltage gets really low.
The last thing to be aware of is that a 30 amp breaker does not trip the instant it reaches 30 amps. These breakers have a standard inverse time-current characteristic as shown in the graph below. More readable image here. A breaker will continue to operate longer under a small overload and will trip faster under a higher overload. For example, the graph shows that at 45 amps, the breaker will run for somewhere between 100 & 400 seconds before it trips or for approx. 200 - 1,000 seconds at 38 amps. What this means is that you can run a MW or a toaster for a minute or two and be fine. However - depends on the age of a CG pedestal breaker and it's use/abuse.
If you go for the 20 amp power inlet, you might consider installing a 20 amp SPDT switch and a dedicated 15/20A T-slot recept. inside somewhere. This way, in cooler weather when not using the AC units, you have the ability to plug in a high wattage appliance and not worry about tripping a 30 amp breaker.
Caveat: Staying at a "modern" CG/RV park (done to more recent NEC code) and using a separate 20 amp power inlet to feed an AC unit is still not a guarantee that you will have no issues with running 2 ACs, but it will greatly improve your chances of having success. Choosing a site closest to the park's main electrical service (or near a pad-mounted transformer) can also help (less voltage drop). Also a good idea to check the pedestal voltage beforehand - if it's already marginal, it's only going to get worse when you plug in. Having a permanent voltage monitor inside is a good idea. I have an LED readout in our living area and can see it regularly go up and down like a yo-yo.
