Another cheap and effective thing to do is add a battery cutoff switch if you don't already have one (or making sure you use it, if you do). These modern campers have much higher "house load" than you think - standby current on built-in radios, TVs, LCI OneTouch, and so on all added up to 350mA for my coach the last time I measured it.
That's nothing compared to running your furnace for an hour on a camping trip. But stored for 6 months * 30 days / mo * 24 hrs / day that's EASILY enough to totally drain a battery. A cutoff switch is like $10 on Amazon. I'm personally a big fan of solar, but you may find you don't even need that if you don't have any drains...