I have a Craftsman trickle charge battery maintainer that I have used to keep my batteries charged and fresh. In fact, it brought a completely dead car battery back to life that I used for two years in my '65 Chevy Van. Apparently it un-sulfates them. Just bought a used TT that had two 6v's that were working but were almost out of fluid. Added some distilled water and have had them on the trickle charger for a few weeks now (alternate charging them). Will be interesting to find out how good they are now (they were made in 2013).
So, note to self, when storing TT, unhook battery.