It's not hard to drive the truck and camper with a broken arm, just you better go slower. I did it last year.
I don't know if you can come up with a number that things will freeze inside as there are so many variables. I know I have gone down to 16 in my Bigfoot and the water was fine. I forgot to turn the heater on. I thought it was cold when I woke up. I live in Alaska and have never winterized the fresh water tank. I do winterized the rest but I always figured it was a big tank and a few cups of water left in it would expand but there was plenty of area to expand to. My real problem area is my fill line. It is already difficult to fill in normal circumstances and if it is cold it won't fill. I have used my water system down to 10 degrees after that I winterize and remove the doors from my cabinets.
