Looking for advice. We have a 2000 National Tropi-Cal on Ford chassis with 54000 miles. On secondary roads at 40 mph it has developed a very noticeable shake in the steering wheel, run it past 40 and it stops. There appears to be no problem on interstates. The coach does have Air-Ride bags front and rear, I normally run 40psi in front. Thought it might be a ball joint or shock going bad but thought I would ask first. Thanks Bill40 ???