First things, before spending one cent on suspension upgrades.
- Load the coach for travel and take it across a CAT Scale
- Set the tire pressure to the tire manufacturers recommendation based on weight
- Get the front end aligned
After doing 1 through 3 take the rig for a ride on the interstate (at least 150 miles) and determine what it is about the handling you don't like. Then and only then can you make informed decisions on what mods you may want to make.
Bottom line, make the mods that you think are necessary not one based on what other folks thought were necessary for them.