Think of amps as "tons of load" your RV is designed to carry a max load of 30 tons (amps), it might in fact carry only 28 or as much as 31 but the design is 30. Anything over 30 will trip the main breaker, or cause other operation problems.
Your trailer can't carry 50 tons (amps) so don't try. Having said that I "think" there is an adapter available that will allow you to plug into a 50 amp service but it steps the load down to 30 amps. Never really saw the necessity in trying to use something like this as almost any pedestal that has 50 amp service also has a 30.
The adapter for the drop from 30 to 20 is something that is practical to have/use. I have one that I use when my trailer is parked at home, it allows the battery charging system to stay on top of the batteries as well as allowing the shore power to feed the slide out operation or the refer/freezer if I want to fire them up a day early before leaving on a trip.
Depending on the BTU rating of your AC unit the 20 amp may carry the load. I think the smaller AC units pull about 15 amps, the larger 18-20 or more.
Having said that I would definitely not use a 20 amp service to operate the AC or even the electric water heater. If there is a weak point somewhere in the system (think also outside the RV) this borderline full load will find it, however for the uses stated above it works great.
Not to throw in a sales pitch but something I have found to be very useful is a the unit linked here.
http://www.progressiveindustries.net/ems_hw30c.htm
Good luck
Tinbender