[JRTJH]

All deep cycle batteries should not be discharged below about 50%. If your batteries are "dead" every morning after not being able to run the furnace all night, you're probably repeatedly discharging them beyond that 50% recommendation, shortening their life even faster.

Sort of "between a rock and a hard place" with the current batteries. So, it's not "IF" but "WHEN" you upgrade, be sure to consider larger batteries that you won't subject to discharging below 50% with typical use.