We've cold weather camped for years in all 3 of our previous TT's and now our 5er. Our current 5er does not count toward your question, as it's a 4 season, good for zero degrees, provided the furnace stay on.
However, our previous 3 TT's were not so robust. But we winter camped anyway.
The way you do it is to go ahead and winterize the camper and then absolutely no water goes down any drain, nor does any water go into any tanks.
Fresh water is carried in in jugs, or 5-6 gallon containers, (or in our case a 35 gallon tank). Dishes are washed in a pan (not at the sink so the temptation to dump down the sink is eliminated). We keep a port-a-potty in the bathroom that has no dependency on running water, and dump it every day into a pit toilet, or (at home) into a running toilet somewhere.
Basically, you use your camper exactly the same way you'd use a tent. Tents don't have water, but a water station is usually set up on a picnic table. You do the same at a designated spot inside the camper. Used water can be tossed out the door, if this is acceptable, bathroom waste goes into a port-a-potty and dumped appropriately.
Hot water? Heat water in a pan on the stove top. Washing? Take cat baths with that hot water from the stove. Baby wipes too.
Pile laundry and do it when you get back home.
You just have to use your camper like a glorified tent, no water in the system at all, and have a great time!