A few years back, DW and I spent a couple of weeks in Yellowstone National Park in Sept with no hookups. It got down into the 20's at night. We learned the first night that even with double the batteries you currently have, we could not heat the RV into the 50's or 60's all night. The fan ran the battery down in the middle of the night. We had to set the furnace to 45 at night, just enough to keep pipes from freezing, and use quilts. The next morning, we would jump up and turn the furnace up to get comfortable. That allowed us to go 2 days without fully recharging the batteries. However, like I said, we had double the battery capacity you currently have.
