I do not use campgrounds. I have camped in 9 below with daytime highs of 15. We had no issues, other than my extra water jugs freezing solid. We did not bring them inside as we were negligent in watching the weather forcast.
My TT is not stock though. It did come with a heated enclosed underbelly, which I improved on by adding another heat vent from the furnace line. I also insulated the entire underbelly with 1" thick R7 insulation. Even the inside of the frame rails. Basically the underbelly is now a large cooler, which has hot furnace air blowing in at 2 locations, one on the fresh water tank. I also insulated all of the plumbing and installed heat tape on all of it. The heat tape manufactuer claims It is good to 45 below. Never been that cold, so I cannot vouch for that.
When camping in exreme cold, we use the propane furnace and generator power non stop. We have a LOT of gas and propane on hand. It is some work, and we are out there for a reason.