I see you have a Southwind .... while ours is older (01 35R) I would think they would be basically the same.
All of our fresh and waste tanks are in the enclosed basement area. While not as cold as you folks get, we have been hanging out in the Denver CO area since early December. Spent one night at Cracker Barrel without any hookups when it got down to 4*F. Another night with electric hookup it was -4*F.
On our unit the furnaces also heat the basement areas with the tanks, the rear furnace takes care of the utility bay/waste tanks at the left rear. The front furnace takes care of the center section with the fresh water tank. I have wireless remote thermometers in each bay to monitor the temps.
For example, last night the lowest outside temp was 18*F; fresh water tank bay lowest temp was 47*F; utility/waste tank area lowest was 38*F; furnaces were set for 65*F. And that 38* could have been while I was hooking up after we pulled in last night.
The manual says something like 10-35 degrees outside set furnaces to 65*. For every degree colder set furnace 1 degree warmer ... ie 5* outside, furnace set to 70*
The only two times I had a problem with freezing up was when I let the low temps get ahead of my getting the furnaces cranked up. As a side note, it appears that when this happens obviously applying heat but also driving will get things flowing again.
So check your manual and/or crank up the furnaces for a while and then check the temps in the critical bays ... I would think you would have heat down there.
As long as I don't let the cold get ahead of me, I'm comfortable with temps down to 0 or a little lower ... it does stay nice and toasty inside as a result and you will use a lot of propane at those low temps, but certainly workable for what you want to do. (on those cold days noted above we used 8.6 gallons of propane in 2 days
, while normal useage tank will last about a week or so - ie 16-18 gallons of propane. This is without any add ons such as reflectex or bubble wrap for the windows, etc ... that makes a huge difference we found)
And we do run the furnaces as needed while traveling, also hot water tank.
So hope this helps ... fire up those furnaces and check those bays.