Is the air ride in the truck portion typically removed on conversions? So you remove it and then end up with a truck that has a harder ride?
Why can't it be left on the truck?
Does the conversion cabin portion ever have air ride?
Have you ever looked at the way a passenger train sections join together? There is a door on each end and the trains join by an 8" wide rubber boot. It doesn't keep everything out, the door is the next barrier, but it must help.
"I have marveled often at the thin line that divides success from failure and the sudden turn that leads from apparently certain disaster to comparative safety." Ernest Shackleton, Antarctic Explorer, Sea and Land, 1874-1922.