What to use to keep battery charged while sitting?

I have a battery disconnect switch. I installed it in the negative battery cable. It shuts off the power to everything when I turn it off. My rig sits for 3-4 months in a storage lot. When I power it up I have never had a problem running my electric jacks etc. The battery will easily hold a charge that long with nothing connected to it. My boat batteries are the same.

B.O.