Get some extra pieces of the siding and conduct some impact test (aka hit it with a hammer) with and without backing. For backing you can try 1/4" ply (but it'll add weight) however I'd also try out some various foam board, they'll provide some backing strength and insulation as well.
Where in WA are you? I've got 25psi and 60psi foam board that I could give you some scraps to play with. I also may end up with an extra window or two from my build if you're scavenging for parts. I know I have a 24"x22" dual pane picture window that is scrap (has some scratches on the exterior glass in one spot).
For the cable lift verse jack stands. I'd plan on having provisions to add jack brackets later on if you wanted to no mater what you do (so put some attachment points on your frame). The cable lift idea is cool but limits you to only being able to remove your camper in your current garage, that is why I'd build in flexibility.