If the solar charger is causing problems, it will affect "maintenance
free" even more as they cannot tolerate high charging voltages.
I would use wet lead acid - cheaper to replace than others and get a
solar regulator so that long term float voltage is not too high - no
more than 13.8v for wet acid cool temp. Reduce if ambient temps are
high - above 70F. Batteries do not like heat. It is not the current
from the solar charging that is the problem, it is the voltage.
Gassing starts at 14.4v.