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