30 amp service

Did I miss something? The OP is dealing with 30A and 15A source to power a 50A coach. A cheater box deals with two 30A sources to provide power to a 50A coach. Not the same thing!

I bought one of these from Progressive Industries years ago and the few times I could use it, it worked slick. Just cannot use very often anymore.

Cheater box

I have one, Like JSO said I have it to use when there are 2 30 amp circuits, Which I have used successfully, I haven't tried it with a 30 and a 15 or 20.

I have, and have used one of the splitter, (called power thieves), and if necessary, will probably use it again. However, be very aware that in many campgrounds with only 30A outlets, the underground wiring supply was and probably still is, designed and sized to support 30A loads. This is why you see so many that have burned out plugs, and possibly low voltage problems. When people try to pull 50A through wiring and breakers designed for 30A, something has to give. Usual symptoms are overheated burned outlets, and lower than normal voltage, which creates more problems and probably damage. You can easily run a 50A coach on 30A, if you just don't try to run everything at the same time. If you have to run everything, then only go to places that have good 50A service. A good rule is that on 30A, you can run two heavy loads, such as an AC and a hair dryer, on 50A, you can run 4. Get a plug-in voltage meter, and watch the voltage. If it drops too much when you use something, you are overloading the master circuits.

i bought ours to use in a specific campground when we had an RV with 50 amp. Laura Walker State Park in Georgia had twin 30 amp plugs at the pedestal along with a duplex 15 amp GFCI. It allowed us to run both a/cs along with fridge and WH on electric.

As mentioned, often the 30a plug is the same line as the 15a standard plug line.
In that case, this device MAY only help if the 30a breaker is weak from old age, and actually trips less than the 30a its supposed to. This is what happens when a 30a plug always trips when the AC turns on, or the AC and the microwave.

Another problem, is what if the 15a side trips, you are back to the 30a side only or vice versa.

The only good mixing of connectors I have found is the 50a to 30a adapter for my 30a camper plug. Even if the 50a breaker is old, and no longer rated for 50a, I have never had a problem getting the juice I needed @ 30a.

They work like your neighbor said, if the circuit isn't ground fault protected. Almost every campground you will go to is ground fault protected.

If the place you are staying now does not have ground fault protection, it has old and substandard electrical service.

A bad idea IMHO, and not very useful in practice. They basically put the power from the 30A on one leg of the 50A line and the power from the 15A on the other leg of the 50A line.

Why is this a bad idea? Let me enumerate some of the ways...

1. As was said, this will not work with a GFCI protected 20A (or 30A) receptacle, such as has been required by the NEC for camp sites for quite some time. The number of places where these are usable is rather slim.
   
2. In the event of a faulty neutral connection on the 30A side, you could end up with 45A going down the 15A's neutral line. That's a significant overload and fire hazard.
   
3. With miswired outlets, you could end up with live hot wires on one of the plugs when connecting or disconnecting.
   
4. With miswired outlets, you can create a hard short when plugging in. Hopefully this will trip the circuit breaker.
   
5. At least in some cases, the 15/20A socket "shares" the circuit with the 30A socket, so you don't actually get any additional power available. (The 30A breaker would be a master breaker for the pedestal in such a setup.) I don't know that this is a super common setup.
   

I've heard of them and they can be found easily enough on the internet and some RV parts places. I suppose if they did not work, they would not be manufactured. I've never had the experience with one myself. I can only repeat what someone else has said, they do work. How well? (That I don't know).

Found one here: Click here.