Perhaps hail damage? The repair shop probably didn't want to install the fiberglass roof, and the owner didn't want to drive back to Iowa so went with the rubber roof instead? If the owner was a full timer, time could have played a roll in choosing the roll roofing too. Just checked what a roll of
rubber roofing on Amazon would cost and it's surprisingly economical at $550...not to mention how much easier it would be for a shop to handle it.
I remember reading on the iRV2 forum a couple years ago about people having their fiberglass roofs damaged by huge hailstones and letting their insurance take care of it. I'd wager your RV is one of those.
I've seen videos of replacing a rubber roof and if it's done in a shop with headroom, plus having a small fork lift, it's not a big job.