Don't forget that many RVs have drain lines extending below the enclosed area.
We generally don't un-winterize ("springerize" - that's a good word!) until we see night temps in the high 30's. We just use bottled water; and antifreeze to flush the toilet in normal cold weather.
I have to admit, that as we came home from FL in late January and the temps were in the single-digits, we adopted a "no liquids down the drain" policy, after I read that urine can freeze around 28 degrees. A bucket, a trash bag, and some kitty litter handled those middle-of-the-night trips...... not elegant, but it works.
Ed, Deb, and 2 dogs
Looking for a small Class C!