There is some water left in the tank when draining... how much? I do not know. But I have never worried about getting every last drop out before winterizing.
My normal MO was to open the hot water tank plug on the last day of every camping trip to let any water that was in there to escape by virtue of the motions when driving on the way home.
When I would winterize, I would close the by-pass valves, pump in the RVAF and screw the plug back in only a thread or two just to only hold it in place to inhibit insect from trying to get in.
Sold the TC, previous owner of 2 NorthStar pop-ups & 2 Northstar Arrows...still have the truck:
2005 Dodge 3500 SRW, Qcab long bed, NV-6500, diesel, 4WD, Helwig, 9000XL,
Nitto 285/70/17 Terra Grapplers, Honda eu3000Is, custom overload spring perch spacers.