Bill is right, never lower your ride height below the specified setting to improve the ride. The noise is probably caused by either a shock absorber bushing or one of the bushings on a track rod or trailing arm.
Do not try to repair the suspension yourself unless you have some experience in suspension repairs as it can be dangerous. Most but not all shock and suspension arm bushings are replaceable without replacing the part itself, but the part will need to be removed to press in a new bushing. Some suspension arms have molded in bushings, so they have to be replaced if the bushing fails.
Larry did not say which chassis the coach has. The 98 Marquis was built on two different chassis. The first 25 or so were built on a Gillig chassis, and the rest of them were built on a Magnum chassis. You will need to know which one you have to repair it.
If any of your shocks are leaking oil it is bad and should be replaced, other than that, I would repair the noise first, as that could possible be a safety concern, then drive the coach to see if the ride is satisfactory after the noise is repaired. Too often owners replace shocks in an effort to repair an unrelated problem.
Gerald