Prevent water from freezing?

After the water heater put a valve. A turn of the vale redirects the water flow to pipe B. Pipe B dumps back into the water tank (or the fill line that goes to tank). This way you can heat the water with propane and cycle it through the fresh water tank, water pump and boiler. Turning valve only half will allow some pressure to escape to the faucets at the same time it is circulating, so you can have some water at the sink while you circulate warm water to the tank.
It will NOT need to circulate constantly, so you manually turn this on and off by guessing how warm the fresh water tank is staying (or feel the "cold" water from the sink).
Because it is open to the tank that is open to the outside air you will never build pressure so the pump will run constantly.
You will use a lot of propane, but can keep the tank outside from freezing.

We used our Hi-lo hunt camping in low teens overnight, and about 32 tops during the day. I just fully winterized the water system. If I didn't the one thing I would worry about most is the toilet fitting in the rear bath. That is as far away from the heater and even a little bit of water frozen in that valve on the toilet will crack it and put your whole water system out of commission. Then it's a crappy job to change. Anyway so we just winterize the water, turn it off, and use those plastic jugs to hold water to wash or flush toilets. You can dump some RV antifreeze in the black and grey tanks to give you more protection especially if you put it in first so it gets a slug down in the drain valve fittings. We decided to just leave the grey open to the ground, it was in the dirt middle of nowhere, and were able to avoid using the toilet at all as well. It was hunting camp after all, be the bear.
For night at first we tried to keep it in the 60's inside the first night but the heater almost ran continuously, so we lowered it to 45 and it still was comfy and the heater ran about 1/4 of the time. We stuffed pipe insulation cut lengthwise in 1/2 all around the upper/lower gap as well to help with drafts but overall the camper doesn't have much insulation.