Go on Costco.com the Reviews on those interstate batteries are the worst most are lucky to get 1 year out of them. Plus 2 is not enough you need at least 4 batteries. Also note since you ran your batteries so low you have likely permanently damaged them. you want to prevent discharge below 12.5 12.2 at the lowest otherwise you damage the plates.
Read the reviews
https://www.costco.com/interstate-6-...100476406.html
Also that 200W heater you mentioned if that is a resistive heat heater NEVER use those on battery power, solar or Gen. Resistive heat is hard on those electronic power supplies.
Buy quality Trojan Batteries or for 4-5 times the cost buy Lithium.
I just upgraded to 4 6V wet here is the post with pictures
https://www.montanaowners.com/forums...5&postcount=31