What people say about how long it would take to freeze in the tank is true. But the lines out of tank can freeze pretty quick, and if the line is full of ice your better off with MT tank. You can't use the water anyway.
I camp in a pop-up camper, and have stayed out when day-time highs where in 30s, nights in teens. My tank is under floor, right at axle. I have 3 pieces of line about 20 feet long with half of 1 of the cheap space blankets secured near the end of each. Throw line under trailer, then pull blanket under. Use magnetic clips to hook to frame, so I have a triangle shaped space between ground and tank. Hang a heat lamp, like what they sell for chickens, (200? watts) on the water line. Cover the end with 1 of the things sold to put on windshield to keep inside of car cooler