Fresh water tank under camper-Camping in colder weather.

Define colder weather. Below freezing at night and above freezing during the day, the tank isn't going to freeze up.
Can't speak for the rest of the camper below freezing.
If there's not a low point drain, pumping our as much water as you can will be ok as long as it doesn't leave, Idk, more than 1/2" of water giver take.
If you do need to add heat, I'm sure there are heating pads you can stick to it but they need constant power.