I've always assumed it will not self drain. When I drain my water system down, ideally when leaving my last site of the year I use the following method.
Firstly I have to admit that I've fitted an isolation switch to my pump. This is a switch cut into the positive feed to the pump, meaning I can leave the 12v system live for lights, but turn the water system off and open everything up.
Pull bung out of bottom of tank, open all valves on my Truma electric hot water heater, flush toilet until no water comes out, flick pump isolation switch, open bathroom and kitchen taps in central position, drive home and hope anything in the system will drain out.
I usually found some water has wept back into the tank, so I dry it out with some kitchen towel.
Some members have had success with pulling an air filled balloon over the end of the taps to blow water out.

Colin

I'd love to see a film of the process. Now there's a challenge for you Mike.

Colin