The underside of the floor of my Palomino is unpainted OSB, and the drain is right at the front of the camper, so draining with the camper in the truck is pretty much a recipe for a ruined camper.
What I do is take the head off the shower and pump the water into 5 gallon buckets to drain the fresh tank. After a trip, that's no more than about 1-1/2 buckets, and takes about 5 minutes.
The little bit of water in the tank is not enough to cause problems if left to freeze, but I do generally dump the last little bit after I unload the camper for the winter.
