A few points to maybe help you on this.
First, I totally agree with you about buying new, especially now during Covid, many new campers built today are not anywhere close to what Sunline offered.
Next, I do know an amount about the fiberglass campers, the vintage Sunline made, and I know of the new "vacuum bonded" wall make up of the modern day ones. Both vintages have the same/similar issues as the aluminum sided campers when a roof leak happens. Gravity takes over, water runs down as far as the volume which entered the roof has to send it down. The point I am spelling out is, if you have 4 corners on a Sunline with the budboard roof, odds are high, a good amount of water made it into the attic of the camper to create enough volume of water to deteriorate the budboard the creating the rubber band feeling. You never said how large the rubber band feeling was (radius from the corner) so I cannot relate any better to how bad, bad might be.
There is nothing in the build of a Sunline to keep water in the attic stuck up in the attic if the volume was large enough. That is the unknown, what volume of water came in? The water many times (most actually) weeps/flows down the corners of the camper. It is an easier path to flow. If the leak was ongoing long enough, the volume of water made it to the floor line where it was stopped by the black Darco membrane and can't get out. Odds are good on a non slide Sunline, the floor area at the back wall and front wall (since all 4 corners were wet) have some level of water damaged wood as the water festers in that area. Corner molding, window to siding seals, cargo doors and the list goes on add to the problem even if the roof did not leak.
Odds are favorable there is some level of water damage in the walls. Why would the damage stay up in the roof area? I suspect front and back walls have some level of damage, and if the water volume was large enough, then it gets into the left and right walls. There are screws along the very bottom piece of siding of the front and back wall that tell a story. If those screw heads are rusted, water was in that wall. If the head is still white, but the threads (have to remove the screw) are rusted, then water was in that wall. If the threads has goo on them, active rot is in that wall.
There is a way to get a good read if there is water damage still there, if it is still wet, which is likely. On a filon sided camper, the moisture meter I keep telling you about, can scan the inside wall of the camper, and the outside. If the water trapped in the walls made it out, then it could leave dry rot fugus that later turns into dry rot dust. The meter cannot find the dry rot, but it can wetness. Not sure if you saw this file in our FILES section, it has a T2363 shown in it that had a roof and corner leak and the moisture meter readings. https://www.sunlineclub.com/forums/d...o=file&id=5638
It would be worth it to scan the walls and floor from the darco and see how good or bad they are. I have seen one Sunline with a big wet roof area in one corner that did not take out walls. So it is possible. But most I get, have large damage.
Henry does have a point on the filon sided campers being different then the aluminum siding campers to repair. They are restorable, it is just that the left and right walls will be very different to deal with the siding, if it is wet behind it. It is back to, it depends how much water made it into the wall. The front and rear walls are most likely made different, easier to remove, repair then the left and right walls. You may need to make new walls, new filon bonded to new laun plywood, then glued back to the wall studs after repairing the studs (the entire wall). Removing the left and right walls you may not be able to salvage the siding. And the front and back is a maybe. There are epoxy resin kits that you can try and re-glue a delaminated siding back on, on the camper, but again it depends how bad the original luan backer board is gone or not.
When I do a camper restore, I chase the water damage to the end as I want the camper to last many years, 15 plus years and is better then new when sealed up. But that does take time. I did finish my 2004 T1950 and it was even camped in by my family this summer. I just never made it yet to posting the last of it. I will in time. That was a total restore that I put a little over $5K in parts into it. There was over 700 work hours. Granted the one you are looking at, may not be as bad as mine had heavy water damage in the back from a roof leak and took out the floor.
One last thing, I recommend anyone looking at buying a used camper, get a moisture meter, and learn how to use it before you even start looking. The seller may have no idea what they are selling has water damage in it. If they cannot see it, then they think it is not there. But, if you have a meter, and learn how to use it, you can show them and they may drop the price, but at least you know eyes wide open what you are buying.
Hope this helps