As an FYI, a July 2004 build date, might be a 2004 or a 2005 model. July is sort of on the edge of being either one.
Your T299SR being a slide camper was built a little different on the rear wall then non slide campers. But not by much. The slide campers have a well so to speak at the bottom of the rear wall as the floor is higher up off the frame and the siding goes below the floor. That well helps save the whole rear floor from rotting out sometimes. Water collects in the well.
To your back wall and how to take it apart, I'll pass along what I know and what I suspect. If and when you get into this, it would be very helpful if you can post back with pictures of how your camper is made. You may be the first to post how your wood studded filon sided camper was made.
What I know.
Sunline made their filon sided campers very similar to the aluminum sided campers with the exception of the siding and how it is attached.
To remove the back wall, you will need to remove both the left and right rear corner moldings. They start up on the roof. When you dig out the vinyl screw strip cover, it will expose the screws holding them on.
Use a heat gun to warm the putty tape and with a totally dulled up (no sharp edges) stiff metal blade putty knife, work it under the modeling. I start at the top, gently pry and heat at the same time the molding and molding joint. Do not stop in one spot for long with the heat gun as it will affect the rubber roof or the siding if left long enough. Keep the heat ahead of the peeling off molding and work it until it comes off totally.
The rear window or much of anything else on the rear wall, comes off the same way. Windows have the screws on the inside flange that has to come off. The DOT lights are screwed in from the outside.
There is also a roof to rear wall molding that has to come off. It is a 90 degree molding, screws on the roof, angle edge over the rear wall. And a molding strip on the bottom of the rear wall that has to come off.
Once you get all the molding, DOT lights, windows out etc., using a plastic stiff blade scraper, dig off any putty tape left on the siding. I find these scrapers work really well for this and will not scratch. They just do not work well when you have to heat and pry with a scraper, the plastic bends. https://www.harborfreight.com/4-piec...set-95832.html
What I suspect.
I am not 100% sure, but fairly sure on these items.
You "may" find staples in the siding you can see at the outer edges where the moldings covered. Dig through the left over putty tape to see if they are there. Check all 4 sides, top/bottom and left and right. And around any window openings. You have to dig those staples out.
Next is going to be an unknown on a wood studded camper. I suspect the exterior wall siding structure is made from 1/8" or 1/4" luan plywood glued to thin filon siding. That is the older way many of the RV industry companies did it at that point in time. The year 2000 & 2001 brochure states 1/4" luan but that may only be on the side walls. Front and back may be 1/8" or 1/4". Or only 1/8" on the front as they have to bend it over the curves.
Filon siding method 1
Sunline may have glued the filon to the luan separate off line from the being on the camper. Meaning the entire bonded filon outer siding wall structure was placed on the wall studs as one big subassembly. If this is the case, then once you pull out any and all perimeter staples, the whole back wall siding will come off, or it needs some prying. They may have put glue on the studs to bond to the luan. Where water got to the glue, that bond is gone. Where the wall is dry, it may have cracked off or if still glued, it needs to be pried off to break the glue bond to the stud.
Filon siding method 2
The other way the RV industry of the time did this, was to staple and maybe glue too the luan sheet to the wall studs first. Then glue on the thin filon to the camper. This means at every wall stud location, there are staples under the filon in the luan that you cannot get to. If this is the case, I would start with wide bladed thin metal pry bar and work it behind the wall studs. Pry gently and work your way up the wall, popping the staples through the luan as you go. If this method was used, odds are high both the filon, and for sure the luan has to be replaced as the staples where just ripped through it.
I suspect they used method 1 as it is easier and a better way to clamp the glue bond off the camper. But, method 2 was done by other camper builders of that time era also.
It is also possible that method 1 was used on the front and rear walls, while method 2 was used on the left and right sides. Or they used the same method which ever it was on all 4 sides.
We have member Draughty, who did some real heavy repair on his filon sided Advancer. This may help, but the Advancer was special as it had aluminum studs and staples would not work. They used a really good construction type of adhesive to glue the luan to the aluminum wall studs.
Hope this helps and looking forward to what you find out.