Having had a membrane roof on a TT and a fiberglass one on our View, it is no contest. Fiberglass is the way to go.
One piece is advertising words of art. Yes it is "usually" one piece fro fromt cap to rear cap on shortish RVs..... but it has a bunch of holes cut in it for the A/C, vent stacks, moon lights and sky lights, fans, antennas, etc..
It is not about "one piece", it is about how well it is sealed.
Modern WBGOs have rolled edges that come down over the sides and join at a small gutter so water runs off the side below the top/side interface.
You want to be VERY careful getting on top of a FG roof. I use big fat bathroom/kitchen rugs grippy side down to spread out pressure points of feet and knees. If you are uncomfortable working above ground....stay off the roof!
Old Crows
2014 View Profile
2010 Four Winds Ventura
2005 Trail-Lite C-17 Hybrid TT