Here is a question submitted to OutsideSupply.com regarding RV Solar Battery Maintainer.
Q: We always leave our RV parked from May through August. The lot we park at does not have power. Every time I go to the RV at the start of the season the battery is dead, what can I use that is reliable and durable?
A: This is a very common problem with RV, Travel Trailers, Class A, Class B, and Class C campers. The battery in the vehicle often drain without some charging. It often take several months to drain on its own but this problem can be eliminated with a small RV Solar Trickle Charger. We offer a few different models of solar chargers that are perfect for this application.
Note these are not really strong enough for going Boondocking very much, you may want to look into a full rv solar kits for that purpose.
Here are a few panels I can recommend: