Integral fuel tanks, like those on RVs, are generally riveted together and made leak-proof using a polysulfide sealer. (Commonly Pro-Seal.) This two-part chemical sealer is bad-smelling and known for getting everywhere, but does a good job of keeping fuel inside—most of the time. The problem comes with rivets—all of those little holes in the tank just waiting for a tiny spot that wasn’t sealed well to give way.