On my boat I use a 400MA solar charger all winter. I think it was like $50 years ago. The battery lasts the normal 4 to 5 years sitting outside in the cold. So I think it does something good all winter here in Wisconsin. In the spring I top off the battery with a “Battery Minder Plus” for a day or two and the boat is good to go.
On my camper, it has a three stage charger for the house batteries that works even when the batteries are disconnected from the house. Just plug in the AC for a few days each month and all is well.
Now my, new to me last year, chassis battery died last winter. So this year I am putting a “Battery Minder Plus” on that battery. It will work when I am charging the house batteries.
I hope this helps.