When I need to go down under, I use 6x6 blocking. each aprox. 18"long. I set 4 pcs. one way with another 4 pcs layed cross wise on top with another 2 2x8s running perpendicular on top of the 6x6s.
then I use assorted blocking to make a ramp to drive onto the 6x6 blocking.
After the wheels are on the 6x6s, I place more blocking under the hydraulic levelers.
Next I dropped the air pressure, making sure that the bags are fully deflated. I then check that all blocking under the tires and jacks is square and true.
I check the jacks and their blocking.
At that point I crawl under to perform my PMs.
One thing to remember that the air rides/air brake coaches only have spring air brakes on the rear wheels and not on the front. So always make sure that the front wheels are properly blocked if you intend to lift rear axle or remove rear wheels.
Last note when that coach is sitting up on blocks I make sure all wheels and jacks have solid blocking (no cinder blocks) under points of support.
"You never fail until you stop trying!!"
My advice is free but I do consider donations