Electricity is not a good source for heating anything. I got by for weeks with a single 100AH AGM battery and two 100W solar panels by using the battery to power the propane fridge controller and the fan for the propane furnace. We made coffee in a drip percolator on one of the cooktop burners. If we had needed to heat food it was with the propane cooktop and this would apply to baby formula as well.
If you don't want to change our camping lifestyle then the option is to add a generator to your rig and run it in the morning to recharge your battery and at night to heat the RV or stay in parks with shore power (best option overall).
We survived in comfort with temperatures in the teens and with our propane furnace as the heat source which was fine as the cubic feet of air to heat was half that of a bedroom and that was with an uninsulated camper. Better to warm the living space then to use electric blankets.
With the exception of a special low temp Relion battery the lithium batteries will not charge at temperatures below freezing or 32 degrees. You could add a 12v warming pad around the battery but I would put this at the bottom of the list of changes to make.
