Pictures would greatly help us to see exactly but it sounds like you need to loosen the bolts on the rear cross sill and pull in the top, you could probably leave the ratchet strap on and use it to pull the top in until you get the correct margin. This is assuming the bottom is correct, also know that the sides are normally welded to the rear cross sill once you achieve proper adjustments, this is probably so to keep the sides from pressuring out like what has happened to your bed. Hope this helps and try to post a few pics of the issue.
