Fresh water tank under camper-Camping in colder weather.

The problem you will face is that you do not have an "all-season" camper. The water tank is just the tip of the iceberg. Campers made for cold weather camping have ALL plumbing up inside the "conditioned space". Just adding a heater to your tank ignores the fact that much of your water piping likely runs under the trailer hidden by whatever undercarriage covering you have. Either avoid camping in freezing weather or do like many who winterize the RV's plumbing and use the campground facilities.