I plugged it in last night, and it wasn't cool this morning. You know what the first instruction is when troubleshooting? Here it comes:
I took the outside panel off, and sure enough, the plug had worked itself halfway out of the outlet, so I plugged it back in, and 3 hours later the unit is definitely cooling down. (I could have sworn I asked my husband to check that at the park...), the outlet is as old as the trailer, so I expect it just doesn't hold the plug securely. Not to say 100% that was the problem at the rv park, my pride is sticking to that .005% chance that it is someone else's fault.
I feel silly, but this is the best kind of 'fix'! I wish the AC were that easy to take care of.