The roof is made up of heavy corregated Budboard (think heavy cardboard) with a rubber membrane stretched over it. The roof structure is lightweight wood trusses.
It is NOT a walk on roof. You'll want to get two pieces of plywood and some carpet remenants big enough to span at least three (four is better) roof rafters (they are easy to see). Place the carpet on the roof membrane, then put down the plywood.
Search "roof maintenance" there are many threads on it, including pictures.
2004 Dodge Ram 2500 Club Cab 4x4 CTD
2007 Sunline Solaris T-276-SR: Misty Blue/Cherry
2001 Dodge Ram 1500 Reg Cab 4x4 OFF-ROAD