OK, you may have a few things going on with the left rear of the camper. Since you have a 2001 T1950, I suspect the trailer frame is made the same way my 2004 T1950 is made. The 2005 T1950's and newer had a frame redesign, but this same problem exists.
Due to the floor plan of the T1950, the left rear shower and more so, the left rear toilet against the left side wall in the back created a black tank hookup challenge to the toilet. The toilet dump pipe would of ended up going right through the left side main frame rail. To overcome the problem, Sunline jogged the main camper frame.
The jog was created by cutting the main frame rail, which is a 4" channel iron a slight ways behind the rear axle spring hanger. They then jogged the frame outward to the left about 8 to 10" or so and then welded on a piece of 3" channel iron to extend to the rear bumper. That jog allowed the toilet dump pipe to go directly into the black tank and the dump valve piping to just sneak under the 3" high channel iron.
That jog creates a flex point in the frame, and weaker then the original 4" channel iron main frame rail. The actual floor system added strength back into the actual camper frame when it was bolted to the frame. Yes, the wood structure of the floor adds strength to the main camper frame in that left rear bathroom. There are not a lot of options on how to do that floor plan and keep the camper low to the ground, so that was the route Sunline took. Other camper brands in the industry with this same floor plan, did the same kind of frame jog, so this frame jog is not unique to Sunline, it is to the floor plan.
With all that said, see here on my 2004 T1950 project camper I am just about completing a restoration on. This link will drop you into the bumper mounting plate cracks and my fix of them. Look at this pics and then go look at your camper and see if the bumper mounting plates are cracked in the same areas mine were.
I also added some reinforcement to the frame jog while I had the camper all apart. And I then saw with my own eyes how much the wooden floor system actually adds structural strength to that left side frame rail job once a totally sound non rotted out floor system was bolted to the frame. See this link which shows the jog reinforcement and the difference the floor system can make. That is a solid non rotted floor system.
A few other questions for you while you are looking into all this.
1. Are the bumper mounting plates cracked?
2. Do you know if your floor system under the bathroom area has any water damage? If you feel you do not have any damage, let us know that and we can show you how and where to look to tell to confirm you do not have water damage or you do just you did not see it before. Just because you cannot see water damage inside the living space of a camper, does not mean you do not have trapped water damage in the walls or floor. Campers are built very different then the way a house is, and campers leak differently.
3. Look at the frame jog joints just behind the left rear spring hanger. Look and see if there are any metal cracks on the main frame rail right in the jog area.
4. Did you buy the camper new, and if so any estimate on how many towing miles are on the camper? If used, about how many towing miles have you added and what year did you acquire it?
If you find cracked bumper plates and main frame rail cracks at the jog area, please consider posting pics so we can help you and others. The 2005 to 2007 T1950's with the new frame system has developed cracks in the main frame rail and bumper mounts after those campers had many thousands of miles on them. We have two forum members who do lot of cross country towing and the frame flexing has affected both of their campers. They have been repaired and are back on the road. All this is fixable, we just need to see what you have and can help better on how to correct it.
Hope this helps.