Tony,
If I understand what you are saying correctly, I think you are referring to butyl tape that goes between the window frame and the outer fiberglass wall of the MH. You can buy rolls of butyl tape from amazon and just push it in place of the back side of the window frame. Slide the window in and tighten the screws until it squeezes to tape out, and then clean off the excess tape.
Hopefully this will be helpful.