You said you already had the batteries checked but when was the last time anyone topped them off with distilled water and then charged them? If they are the original Harris batteries, they are terrible at holding a charge especially if you are in a colder climate. Also, make sure everything electrical is turned off including water pump, heater, TVs, etc. as they do have a small drain no matter what.
I would recommend using a dogbone to step down to a 20amp plug and keep your coach plugged in at home which should solve the issue until you can figure out what the culprit is.
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