Being your are posting in the PU section. I am sure you do not have a heated underbelly. Some of the PUs now days are really nice and have a lot of the features of the big TTs just in a smaller package.
We have been out in the low 20’s in our HTT. Ultimately the question is how long will it below freezing? If it is just for a few hours overnight, and it will be well above freezing during the days, then we just put water in the freshwater tank, and use it just like we do when boondocking. Absolutely disconnect the water line to the CG hose bib, and drain the hose. We also tend to crack open the cabinet doors that have plumbing running through them, just to be safe. Make sure to run some heat, so the inside stays above freezing and you should be fine for the night. Running water in theory does not freeze, but you have to have enough flow for that statement to be true.
As for heat we like to use a 1500w electric heater, we got it for the first PU that did not have a furnace and it kept us quite comfortable.