IMO, I wouldn't gamble with my own health nor a child's by trying to overcome an RV battery issue by using an unvented space heater. Yes, most are perfectly safe when used according to the instructions but I think the science is yet to come out on what are safe levels of byproducts or oxygen in the enclosed space.
A single group 24 battery is inadequate for RV use when off the grid. Typically, you have 40 AH of usable reserve with a small battery like that.
Second the suggestion to buy two 6V GC2 batteries at Sam's, Costco, local golf cart distributor. You'll have some minor installation details but the 6V batteries are a durable solution.
Good links for knowledge about 12V systems and batteries:
Battery university12 V side of life