I have a fiberglass Bigfoot camper which is referred to as a clamshell as it is made in two halves, top and bottom, which are joined together with the upper half overlapping the lower half. This type of build eliminates all the edge seams of campers made by joining together various side, front, rear, roof, and bottom panels. So in that respect they do have less potential for leaks. BUT they still have plenty of holes cut into them for various lights, vents, skylights, air conditioner, windows, entry door, baggage compartments, furnace, hot water heater, ... you get the idea. Every camper will have these and they are all a source of potential leaks. Inspecting all caulking regularly and addressing any issues promptly
is your second best bet to keeping your rig going ... second only to keeping the rig under roof. But then what fun is that! While my Bigfoot is a slide-in camper which sits in the back of a truck, Bigfoot also makes travel trailers with the same clamshell fiberglass design.
We also have a travel trailer with a rubber roof. It is a "lite" version and one of the ways the manufacturer cut down on weight was to use thinner roofing material. It only took a few years before it started to get holes in it. And the only way to fix it was to replace it with better material at a cost of about 20% of what the unit originally cost. Thus, if you look at units that are "lite" or "ultra light" be sure to ask yourself what was sacrificed to save on weight.