Electric resistance heating is probably
the MOST inefficient way of heating. Don't even CONSIDER trying to heat with electricity if you are boondocking.
If you are going to upgrade you battery, the most cost effective option is TWO 6V golf cart batteries. They will store more energy (210Ah) compared to a pair of dual purpose marine/RV Group 27 batteries (180Ah) and WILL COST LESS !
As for a box,
NOCO HM426 Dual 6V Battery BoxNow if you goal is to eliminate propane all together, the what you need is a custom installation of a residential mini-split heat pump. You will need a big inverter AND at least FOUR 6V golf cart batteries. You will be running that generator during the day to recharges those batteries.