Hmm, that's a tough one. There were five places- stamped into the left side of the coupler, under the door sill, on the weight sticker on the left outside sidewall, and on the quality control sheet glued to the main closet wall, and in any paperwork received with the trailer originally. Most trailers only have under the door and the coupler stamping left. The coupler stamping is very shallow and hard to see with layers of paint, so unless you know the coupler piece has been cut off the tongue rails and a new one welded on, you may just need some sandpaper.
I've heard reports that the door sill tag on the older ones may be more under the door frame than the sill plate. It was literally just a piece of tin with numbers stamped in, stapled to the floor. If you see the edge of it sticking out from beneath the door frame when you have the sill off, that may be it, but you'd have to pull the door to see it.
The quality control sheet was always glued to the back of the largest closet. I'm guessing you don't have it since you should pretty easily see it. It was just a piece of paper, so it can fall apart with age and can be taken down when renovations are done.
The stickers on the exterior have pretty much all faded away now, they didn't weather well. you can probably see a shadow of where it was on the siding, if the trailer hasn't been repainted. It was maybe 4"x4" and about at eye height.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9473.8 (as of 6/18/21)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR