Hi, we have a 1983 t1550 also...more pics of yours would be welcome. When we got ours I found the same type of problem, soft floor. This was from roof leaks. Trailer sat in a field in Northwest Connecticut for several years and was subjected to several feet of snow on the roof. We resealed the roof with a latex rubber sealer used on flat roofs and stays flexible. Knock on wood, no leaks even in the heaviest rain. Had to patch the floor, Cut out sections, replace sub framing and reseal joints. It has a metal floor protector the entire underside of the trailer. You need to be sure it has not been ripped open or rotted out as it protects the underside of the wood floor. All of my windows are louvered crank out style. The last 1550 I saw with your windows was a 1985. Inside, on the back wall of the closet should be a paper "birth certificate" of your trailer. Good luck!
Lincoln