Is the fan blowing at max speed? Is it(fan) making any kind of noise? Could need to be cleaned.....have you changed the cabin filter lately? Has the ac ever been recharged?
A fridge should be able to run off of 2 to 4 deep cycle batteries but you will get longer life and more run time off of 6 volt golf cart batteries wired up as one big 12 volt battery.
You will not run ac off of the battery bank period. You will NEED a generator. depending on the size of the ac you will need at least a 2800 watt generator.
Battery banks are good for espar heaters and some cooking with inverters.
Generators are built to be out on the weather just make sure they can't get taken.
Because your driving someone else's van your best bet is to use a good powerful fan to cool yourself off with.
Would you fly in a electric plane?
The only way I could see a electric plane would be good is if the battery bank could be swapped out at each airport for another flight.