30 amp seems like the only way to go. If for some reason there are no 30 amp pedestals at a cg, all you need to do is use a 30 to 15 amp adapter on the end of the 30A cord and go into energy conservation mode. At home you just need to use the same adapter to keep your unit plugged in if you want.
If using a 20 amp receptacle, you really should be using a 20 amp rated #12 gauge extension cord to the source but I doubt many, if any would in a cg. As long as you plug just one appliance like a toaster or coffee maker in, you'd be okay, but if you were trying to plug in more than that in your trailer, you could end up burning up a 15 amp cord. Many extension cords for 15 amps are only 16 gauge (15A = #14 ga.) and not capable of a continuous 15 amp load.
However, if the 20 amp breaker in the pedestal is GFCI, which many (most or all?) are and you have a GCI receptacle in your trailer, the two won't work together in series. You can find info. on this by googling such as:
GFCI on pedestalBut what RV has a 20 amp service in them?
Related tip: Never plug your cord into the pedestal unless the breaker is turned off. The converter inrush current will cause pitting on the contacts eventually leading to resistance high enough to burn out a plug or receptacle. You can hear a snap, crackle, pop when you plug in live and in the dark, also a flash. That's why you'll often find a pedestal breaker turned off when you get there.