It looks like you have a real "project" on your hands. They are all rebuildable and we have many members who have done some major rebuilding.
The 1992 sales brochure may help as a place to start on your original floor plan.
To get to the files section, you need to be logged into the forum.
At the top of the forum are tabs, look for the word "FILES" click it.
Once into the files section, scroll down to the "Sunline Brochures" section. Go in there and find a sale brochure for the year camper you are looking at and down load a copy.
The spray foam you show in your pics is not original. Sunline used mostly unfaced fiberglass batt type insulation. There an Advancer line of campers in the mid 90's that where aluminum framed, that used an open cell Styrofoam, but is was only for a limited time and only on that early Advancer camper. The rest of the wood framed campers all the way to the last 2007's used the fiberglass.
The floor thickens, I am not sure on your smaller 92, T1850 what the floor thickness may be. On the newer tandem axle campers the floors where 19/32 (nominal 5/8"). Your smaller camper may be less, I would not suggest going any less then 1/2".
One of out members may have some 1992 pictures to share so you can see the basic styles of the time. It might not be your exact floor plan, but seeing still helps regardless of floor plan.
Good luck with your rebuild. We are here for any and all questions.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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