How does Airstream heat the fresh water tank? They design a forced air system that pumps warm air into the tank cavity. Transfering heat this way, through the plastic tank and into the water, is pretty inefficient, not to mention all the heat that escapes as the air is blown out the bottom of the trailer.
A better way is to just put hot water into the tank. If you have a sensor attached to the drains, you can automatically open a solenoid valve and allow hot water to feed directly into the tank. The hot water heater is a lot more efficient at warming up water than the forced air heater. Dumping the hot water directly into the tank is the most efficient method of heat transfer. Automating this with a sensor means that you can open the valve when the drains get down to 34° and close the valve when the temp is up a little higher, say 37°.
Since the water pump is automatic and is pressure sensitive, when the valve opens, water will flow from the hot water heater directly into the tank, with no modifications needed other than a short run of PEX and the valve. And the pump will be pumping water out of the tank, ensuring that the warm water flows through the tank and into the water pickup line. You can design this so the warm water goes into the far side of the tank or near the pump pickup, depending on how much freeze protection you want.
This modification also has the advantage of inducing flow is some parts of the cold water lines, keeping them from freezing overnight.