I have been using a solar panel
BatteryMINDer Model SCC-015 for over six years. I originally purchased it to maintain one 12v Trojan Deep Cycle battery on my PopUp.
For the past year it has been maintaining the charge on the motorhome batteries, or my Ford Expedition, (whichever is being uused less).
Besides including a charge controller, it also desulphates any batteries that
were not well maintained to begin with.