Just to add a little to what mboatwright
My LaCrosse tt had two grey tanks linked together. It had a rear kitchen floorplan and in order to completely drain the galley tank I had to have the front and drivers side of the tt low.
You can also blow the lines out (I believe using no more than 45 psi) after using the low point drains when winterizing the tt. They make an adapter to screw into your city water connection so that you can use a normal tire chuck.
If you feel that you have removed about all of the water in the lines you can add rv antifreeze, or not, depending on how confident you are that all water has been removed.
When adding antifreeze be sure to add some to all your p-traps and you can also make sure, or not, each tank has some in it as well.
Hope this helped.