I would take a look at Eco Worthy or Newpowa or similar import solar panel kits for this. If all you want to do is maintain batteries, and maybe run some ventilation fans, and perhaps run the control side of an RV propane refrigerator, something in the 150 watt panel range should be more than enough. Say $200 - $250 for a kit with mounting brackets and charge controller on ebay. If you don't care about fans or running an rv refrigerator an 85 watt panel would be plenty, an 85 watt panel with mounting brackets, charge controller, etc. will run $110 or so. For ease of setup I would have a separate system for each RV.