Maintaining coach battery while in storage with no hookup?

coleman among other labels make a solar panel that sits on the dash and plugs into the cigerette light socket. My daughter has one and it keeps the mh battery charged. It worked so good that my son in law wired in a plug for the coach battery. Only drawback, they cant's use the windshield cover to prevent sun damage to the dash. They covered that with a large towel. I think they make like a 15 watt, a 25 and 40 watt. Thats all you need to keep battery charged.