Dunno if this still applies, but on my '01/'02 Roadtrek 190 on a LWB Chevy Express 3500, I'm pretty sure that there were no 'helper' or 'overload' springs. As I recall, the clearance was pretty minimal between spring pack and frame. The rear leaf spring pack was pretty beefy, tho'.
From later experience with my Tiger, the RT's ride probably woulda been helped by 5000 lb air bags on the rear. They would have taken some of the 'slam-bang' over things like expansion joints out of the ride. I'm not sure what's available as far as fit, tho'.
Jim, "My get-up-and-go done got up and left."