I spent some time a few years ago looking into solar, and there is a few things that need to be considered when going solar, even with the basic approach you are describing.
I have heard so many different options on sizing an RV solar battery maintainer, trickle charger, etc.. One important thing to confirm is will there be any level of "amp" draw from the batteries while the RV is in storage (LP detector, etc.).. Also, will the solar panel be in a location that will get adequate sunlight and be free from debris (leaves, snow, ice. etc.).
I was once told that a solar battery maintainer should be at least have a 10-15 watt panel, but I never did confirm this. Also, I would imagine that some from of controller/regulator may be required to avoid over charging the batteries.., I'm just thinking out load here.
SunForce is one solar product manufacture that I have run across on some forums, but I don't have any first hand experience to make any recommendations. Click on the "Products" tab after opening the web page: http://www.sunforceproducts.com/
Couple of other solar forum/product source links:
When I was looking into solar power for my TT, the guys at AM Solar were a great resource. I realize that you aren't looking for a full blown system, but they still may be able to steer you in the right direction: http://www.amsolar.com/
Help this helps.