If you're only looking to install a solar panel to top-up your batteries, then I would suggest a single 100 W, 12 V panel from Renology. You can find them on eBay for about $135 including shipping. You'll also need a solar charge controller, like the SunSaver-10 PWM, cost about $55 from AMSolar.com.
Read through everything on AM Solar's site and you'll have a pretty good idea of what to expect. The charge controller will include instructions on how to hook up the panels and connect the controller to the battery. You'll tie the solar charge controller directly to the batteries via a fuse. It's also a good idea to add a disconnect switch between the panels and the solar charge controller in case you have any maintenance to perform. There are some other odds and ends, like a combiner box on the roof, wire, feet for the panel, etc. Most of this stuff you can talk to the folks at AM Solar and they'll help you out.
If you have specific questions, fire away.
2015 30' Classic, "Chez Nu"
2014 RAM 2500 w/Cummins Diesel
ProPride Hitch, 400 W Solar