Done a lot of winter camping in 3 season campers. I modified the water system so i could isolate the sink area and just run cold water to the kitchen sink. Helps to have the furnace near that area so it stays warm, or leave the door open and run a little fan to keep the air moving. Fresh water tanks are usually inside and it takes a lot to freeze them up. I throw some sidewalk salt in the blackwater tank to keep from freezing hard and busting valves. Gray water I leave open and put a five gallon bucket under it. Gets much below 10 below and things eventually freeze. Furnace runs a lot too.
Got a Bigfoot camper this year. One of the best 4 season campers made, but don't come cheap. Two fiberglass shells joined in the middle so no leaky rubber roof to deal with. Winter is hard on roofs! Bigfoot has everything enclosed in a heated basement and are supposed to be fully functional to 10 below. My pickup camper unit has a 30K btu furnace and puts out some serious heat. Looking forward to showering after a days ride this year! lol