We don't have power at our storage site. I started using a portable 100W solar panel to keep the batteries charged up. I didn't have much faith in the cheap charge controller that came with the solar panels as being suitable for being continuously connected to the batteries. So I bought a Bogart Engineering PWM solar charger and wired it to the batteries. Choice was driven by the fact we already had the. Bogart Battery Monitor. The battery monitor tells me how much solar power is being generated and how much power had gone in and out of the batteries.
The portable worked very well over two winters. The SC-2030 is smart enough to not over charge the batteries and I think I've only needed to top up water once over then winter. Even then it was minimal.
This year I installed 200W of flexible panels on the coach roof so I don't have to haul out the portable and have something sitting outside that is subject to being walked off with. I do have a pig tail installed on the solar charger so I can hook the portable in parallel with the two roof top panels.
One unlooked for advantage of having the 100w panel (now 200w) is that if I forget to put the Use/Store switch on the coach into Store I still have enough power to not have coach parasitic loads discharge the batteries. I've seen some folks with small 10w or 20w panels, and am not sure how well they work over time.
