The two lines going thru the floor to underneath are the low-point drains. If you open them and let them drain--open faucets inside to help letting air into the lines--it should get most of the water out. The water tank drain is separate and it should be drained also.
After the water has stopped draining from the low-points, then you could add air and see what else comes out the lines. Don't forget AF into the drain traps under sinks and shower.
If you drain the water tank, that should be enough for it--no need for AF into it--and if there happens to be a small amount of water somewhere in it, then there is plenty of room for expansion when it freezes.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired