When we had it, we camped in our pop-up in cold weather. Coldest was about 5 degrees F. And stayed warm
Didn't use the trailer water system during that trip though.
Another trick is to use two heaters if you're on 30A service, and then you might not need the propane furnance at all. If your trailer only has 15A plug or your breakers/outlets aren't configured for that large of a load, you can run a second 15A plug into the trailer (if you can find an existing hole, through one of the bed ends, or in through a slide seal), so you can run 2 electric heaters. I've seen 3 heaters used before, but I think that's overkill, unless it's -40 or something. You can do this by using 2 heaters on the 30Amp service, and running a seperate cord from the 15A plug on the post to run a 3rd heater. Don't count on the campground post to necessarily handle this though.
If your bed end's don't make a good seal where the canvas meets the structure, I used to use pool noodles shoved in that area on my pop-up to make a better seal and insulate around the perimiter of the bed. Before the pool noodles, I would shove clothes, and towels to stop the drafts.
Heated blankets are nice. I got way too hot using a heated mattress pad, but that's just me. I found with a heated pad, you can't get away from it, whereas with a heated blanket you can always toss it off if you get too hot.
Reflectix is great. Use it everywhere it makes sense. You can even make chair liners out of it to stay warmer by the fire.
Most important though is to stay dry, and have lots of firewood