[Tf175]

I don't know where the valving that controls the air suspension is located on your '98 . On my '86 the valves are in the right engine compartment,  forward bulkhead . Yours should be accessible, you can listen ,spray with soapy water to look for leaks .
Are you switching to travel mode before driving ? 
It could be automatic on your coach .
I have a switch that puts it in manual leveling mode .