Leaking Class C's may be inevitable, however, it may be minimized by constant maintenance. Solid fiberglass roofs are less likely to leak, but still need re-caulking of the seams as do all kinds of roofs. Caulking around windows should be checked regularly, also. Some roofs need to be coated with the proper material yearly. Caulking around running lights, vents, antennas, and air conditioners also need to be checked. It sounds like a lot of work, but does not need to be done in one day. Important that you do not wait until a leak is discovered to check all seams.