Water takes a lot of time to freeze actually. Look at the tiny amount of water in an ice cube tray, and that takes hours to freeze. If it's only freezing for you a few hours a day and the rest of the time is 40s-50s, there's no problem at all.
Twice before I added external heat to my coach, the gray and black tank did not freeze, but the dump valves did. There's significantly more surface area and less volume at the valve than there is in the tank. Having the tank full of 30 something degree water isn't enough to thaw the valves either. Now the valves are wrapped in foil, with heat trace over it, and then insulation and gorilla tape. Heat trace gets switched on whenever it's 35F or colder, and has it's own thermostat to turn on/off as needed.