are you concerned about your HOUSE batteries, or your CHASSIS batteries?
I suppose you are talking about 'while in storage'?
a simple 100w solar panel, wired to a solar controller, and wired directly to your NEG and POS terminals of your HOUSE battery bank might suffice if the panel is in the direct sun most of the day, and you don't have anything of size turned on in the coach
a charger for your Chassis batteries is probably unwarranted
A solar panel does not require any 'charger' since it is providing the charge itself, thru it's 'controller' which simply manages the incoming power, and the outgoing amps to the batteries
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Hiawassee, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )