Here is another thing to consider as you winterize:
The compressed air may work fine for the water lines, including the traps......except for the P trap between shower pan and the grey water tank. Even if a Hepvo valve is installed, I wonder if some residual water may remain in the nearly horizontal valve, which could cause freezing problems.
I just replaced the Shower Slide valve in our 2003 17' LD Casita
, thinking I was all set. But then I noticed that the rubber P trap was leaking from some cracks in the rubber. That could have been caused by aging, or it could have been failure due to expanding freezing water in the trap this last winter, when we had many days of freezing weather, sometimes approaching zero.
I may have to drain the P trap each winter, remove it to avoid a second failure, or pour a few cups of antifreeze directly into the shower pan. Alternatively, I might modify the rubber cup of a Plumbers Friend ("Plunger" to some people) to place it over the shower drain and inject compressed air thru the original threaded hole for the handle.
Just a thought,