Does the pump get that last couple inches out? If not, that seems like a lot of unusable water to always have to cart around.
Angling the RV as navegator suggests is a good plan. Having a bit of water in the bottom of the tank won't cause any problems even if it freezes; there's plenty of room to expand above, and the tanks are somewhat pliable anyhow. I would not dump a bunch of antifreeze in the tank just because of a little leftover water.
(My 1998 Coachmen Santara has a separate tank drain valve, clearly labeled and near all the other valves that are consolidated onto a sort of water control panel.)
