You should also think about lowering your battery usage through upgrading things and conservation.
For example: are your lights LED's ????
They use a lot less power and will help you go for a lot longer. I love my LED light bulbs that I installed in the camper in place of the originals.
I have 2 deep cycle 6 volt golf cart batteries and can easily do 3 maybe 4 days with normal draw (maybe more).
But, I can't run the furnace much. It is a power hog. I try to warm the camper up a lot before going to bed and then set the furnace at 60 degrees. I keep more blankets handy to cover up with as the night goes on.
If using the furnace ; even being frugal with it; I am stuck to about 2 maybe 3 days. Depends on who cold it is and how often it runs.