I agree you need to see what your AHJ has to say about this but I would think that the rules are somewhat relaxed since these are not really living space while they are in storage.
If you wanted to go high tech I would consider a system that cycled the power around the various outlets with X10 technology so in a group of, say 10, only one was on at a time. If you can get everyone to agree this is an "installations under single management, where documented safe switching procedures are established" you can probably use the line cord as the disconnect in addition to a central disconnect.
At that point you could probably get away with a receptacle in a big Bell box enclosed in a little "bird house" type structure to keep the rain water off it but you might still be getting close to the price of the RV load center by the time you are done. I bet if you bought the RV box in bulk you can beat your local price.
