Just went back and looked at your pictures.
mcbrat wrote:
water heater

water tank

I've never seen a water heater like that as original equipment in a popup camper before. I wonder if your camper even had a water heater originally? Is there a spot on the passenger-side of the camper that looks like the siding has been patched over? I don't see any place on the drivers-side or back that it could have gone.
At any rate, it looks like the way it's installed you could drain the heater tank by opening the valve on the side of the water heater that has a tube going through the floor, and opening the sink faucets.
In the winter, you would still want to pump anti-freeze through the water lines, but you could bypass the water heater by taking the in and out lines off the top and hooking them together with a fitting or hose of some sort. Leaving a little water in the heater tank will be fine if it freezes as long as there's plenty of room for the water to expand as it turns to ice. As long as you drain the tank down the faucet level, it will be fine. Same for your fresh water tank: drain all that will come out through the drain tube and the freezing water won't break the tank.
:):)