Fiberglass roofs work fine ad are probably the premier roof but they add quite a bit of weight and are generally not used unless the rig's chassis has plenty of weight capacity to spare. There are also roofs that some people (including RV salesmen) erroneously refer to as fiberglass but in fact are a shiny, hard-finish EDPM rubber called by various brand names such as Britek. These are also excellent.
Next in quality are the soft EDPM rubber roofs. It's only drawback is that it chalks a lot and sometimes gets dirt embedded into the pores of the material. Thus it typically requires a lot of cleaning of both the roof and the sides of the RV. Soft rubber roofs are also suseptible to low hanging tree branches.
All aluminum roofs are generally considered to be least desirable but they work fine as a roof as long as the seams are well caulked.
As for insulation, R-value is the key rather than type of material. Foam may be better or worse than fibergalss wool, depending on the thickness and density of each. Typically, though, fiberglass wool will have a higher R value per inch of thickness. RV manufacturers like the foam because it is very easy to install when the walls are manufactured, i.e. it has a very low labor cost. Weight is insignificant for either material - they are mostly air anyways.