I know from experience that letting your battery just sit in the rig while it's not being used is slow but sure death sentence for the battery. Two of mine died completely in my old trailer, and they were only two years old. I just ignored them during the off times, and they retaliated.
Then I got smarter, and built a solar charging gizmo. It's an A frame of wood, with two 5 watt chargers powered by the big nuclear inferno in the sky. The frame holds the panels, which attach to the batteries by nice leads, and keep them trickled up to charge whenever the sun is out.
It was easy to make, and it just parks on the ground. I have a cable bolted to it and lock the cable end to the rig for security.
I just finished up a post on this, with photos, at MyVoltageRV.com
so click on over and have a gander at it. Feel free to copy if you like.
There's nothing quite as satisfying as building something with your own hands and having that thing save you a lot of money by extending your battery life.