Leave the heat on lowest setting while you drive. Do it all the time in temps down to 0* and have never experienced frozen tanks or waterlines. I do know none of my waterlines touch the outside walls. I only have 1 20# tank and can make it for at least 6 days before I need to refill with still about 1/4 tank.
I have also found that it makes it much nicer to stop about 15 minutes before I stop to turn the heat up. That or at least make sure all of my covers are thrown back off the bed. But if I was traveling everyday and had a good charge circuit to the camper, would put an electric blanket on the bed to keep off the chill from the bed and not worry about turning up the heat at all.