Your analogy with a boat is very wrong!
If you were to invert your fiberglass roof and try and make a boat out of it, you would break into pieces at the first wave.
If you inverted a boat and try and make a roof out of it, you would tip the RV over at the first corner.
Boat construction is heavyweight fiberglassing - RV roof construction is extreme lightweight fiberglassing.
As others have said, the seams allow some flex and cheaper manufacturing and lighter weight. If you were to glass them then you would introduce additional stresses that may result in a cracked roof down the road.
Try it by all means, it would be an interesting experiment.
