Best way to keep chassis battery charged over winter?

I'm in west Michigan. Winters get cold but not below zero for any length of time. I simply insure the battery is fully charged then disconnect the negative lead. My house batteries have a disconnecting relay. In the spring I connect the neagative lead and the MH will start right up.

I have had problems with a trickle charger ruining a battery after it became disconnected from 120 VAC. Also my MH is stored in a field during the winter so there is no 120 VAC source available.