Buy a $20 tube of black windshield installation polyurthane. Clean the area where you suspect the leak, on mine it was the corners. Get some good masking tape to protect the parts where you don't want polyurethane. I used a large phillips screwdriver to hold the gasket open as I followed with the tube tip of polyurethane. When done applying the black goo, I carefully removed the masking tape. No more leaks. On an 18 yr old MH chances are the gaskets are dried, stretched, shrunk and generally in poor shape. The black polyurethane , when nicely applied solves it all. Yes it could be a problem for the next windshield installer, but that is his problem.
2000 Bounder. 2000 Vanguard 17' boat, 5 dogs, 2 cats, 1 miniature ponies, 1 horse, Massey MF65, 2013 Kia Sorrento, 2003 GMC Sierra Denali Quad Steer- 1 longtime patient wife(56 yrs)