I would suggest some driveway camping. You really should know what you are up against before you find out the hard way. It is important that those batteries have a full charge before going. Even though I keep my batteries up between trips, I put a charger on them 3 days before we go.
I will say these threads are interesting. What, you only have TWO batteries and you are going to try to run your furnace? YOU ARE GOING TO DIE!!!!!
Our trailer has had the same 95 ah grp 27 that came with it for four seasons. Our night time temps are in the 30s mid summer and below freezing spring and fall. We run the furnace every night and after a long weekend the resting voltage was down to 12.3v. We were being extremely conservative not knowing what to expect. Now we have solar and are total pigs with the power. The battery is fully charged before sun down and reads 12.57v resting the next morning. We light the rig end to end with LEDs and set the stat ay a comfy 74F day and night. Next year I'll get a pair of 6v so that we can go more than a few days when it is cloudy.
So every rig is different. Get those batteries charged up properly and see what YOURS can do for you. LEDs are inexpensive and make a huge difference.