Some folks call it rubber, but it's the same material used on many flat roofs on stores, etc. On a stationary flat roof it's often covered with gravel to keep UV rays from damaging it, but that doesn't work on an RV. If should be periodically coated with a sealant/protector, and the joints and vents through the roof should be checked every 6 months or so for cracked caulking. Not a hard thing to do, it's a very durable roof. Aluminum roofs are noisy and rivets can work loose. Fiberglass one also need periodic maintenance. The EDPM is flexible, something important in a moving, shaking RV.
We like our RV, it's small enough to get into almost any spot but large enough inside and with enough storage to haul all the stuff we take with us.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI