another battery ?

I have 2008 Forest River Sunseeker class C motor home, thats park and won't be used untill spring. Will it hurt if I leave the electrical plug in all the time to keep the batteries charge or would that over charge them, If I turn off the disconnet switch for the batteries it doesn't take long for them to go dead will that hurt the batteries. The temp will be getting way below 0 degrees. Whats the best thing I should do, Thank's ken
Re: another battery ?

If you leave the batteries on charge, you will have to check water levels regularly. It is likely that your charging system will overcharge them.

Letting the batteries go dead will likely make them go bad.

I disconnect mine with the switch and provide a small trickle charge either with power or with a small solar panel, and I still check the water level regularly.