Fuel tank problems, K3500

My parents have a 98 GMC Sierra K3500, with a 34 gallon tank. Twice now they have had this problem of fuel leaks caused by a crushed and kinked fuel tank, both instances being within 6 months of each other. I know that its a GMC, but its the same thing as a Chevy, and more people probably post on this forum than the GMC forum. A mechanic hypothesized that it might be the straps holding the fuel tank on causing it, because of vibrations. Every day we drive at least 5 miles down a gravel road with washboards, which shakes the truck very much. Does anybody know a solution to this problem, or what is really causing it?

It is possible for the gas cap to cause a vacuum; the pump will continue to move fuel and eventually, the vacuum will collapse the tank. Ray