x3 on solar.  We have several PulseTech 12 amp chargers that are meant for use on equipment that's in storage for long periods of time.  Voltage varies and drops to a float state when needed and ramps up enough to desulfate.  I had one on my rig with 2 6v batteries, for several months last year, and never had to add water.  Here's the link: 
https://www.pulsetech.net/store/solar-battery-chargers/12v-solar-chargers/sp-12-solarpulse-12v-battery-solar-charger-maintainer-12w.html.  Not cheap, but cheaper than killing batteries.
Lyle