On the short side, condensation is the main thing. Depending on how well you are insulated, depends on how the outside walls react. My coach is pretty well insulated, but in the bedroom one of the alum tube structure is not filled with foam as it is an electrical conduit. It transmits cold to the inside wall, this builds condensation on it, and it is so cold that it froze. This was at about 17 below. Temps at about 5below and above it was not an issue.
Try keeping some ventilation. Especially when cooking or showering.
Outside, a heat tape on the freshwater hose with foam insulation plus on the coldest nights, leave a faucet just drizzleing, OR disconnect the hose and use internal tank and water pump. If using the drip, be sure to have your grey tank open and hooked up!
I am winter work camoing at the Grand Canyon and we had a 2 week cold snap with record breaking temps. More seasonal now with nights in 20's, days in 40's. Like being at the beach.