At this point, I'd suggest comparing your actual tire pressures to the door post sticker, and to what other owners are using on similar units. My C is very much smaller & on a different chassis, so I'm no help. But I know some lot jockeys see the '80 psi' on the sidewalls and pump them up to that without reading the rest of fine print. That can cause a minor vibration to become a lot bigger.
Also, some RV makers cut & lengthen the chassis between the axles. This requires lengthening the driveshaft, so it's a suspect... thrown weight, damage, etc. If it's a two-section driveshaft, check the center bearing.
Good luck with it!
Jim, "The buck doesn't even slow down here."