Take this oppertunity to get a larger 12v battery. Go get a group 27 or larger and put that on the camper. You can charge and carry the one you have now as a backup once the new battery runs low and swap it out if it's only once or twice a season.
Or go get two new 6v's or 27 or larger 12v and give the one you have now to someone and you should be good to go for 3 days very cold weather or 5 days or so with moderate furnace use.
Doing so also opens up a longer window of dry camping to places you may not be considering now because of your battery bank limit.