You could try removing the valves as part of your winter prep work and leave them out over the winter. I had a Class C and lost a toilet valve over the winter as well but I used the air method, thought I had it all out as well. I would try pumping AF in and leaving it in, that's my plan for our new TT this year and going forward. While not terribly expensive, it is a pita for it to happen.
'08 Greyhawk 31SS Traded in
TV 2018 F150 FX4 SC 3.5EB TP/MT