OK a 2000 T2670 See here. On the top of the forum screen is a tab that says "Files" click the tab. All kinds of good stuff in there
Sunline Coach Owner's Club - Sunline Owner's Files - 2000 Sunline Solaris & SR Travel Trailers
I'm "assuming" you have the dinette?
This ideally would be a great question for the factory. They use to help big time on these kind of questions
I'll give an "opinion". You will have to figure out if it is what you want to do or not.
Removing the couch, I do not see a problem.
Removing the cabinets up above the couch, not seeing a strength problem.
"Completely" removing the wall, that makes me stop and think. There are no cabinets on the wall to the floor from the bed to the dinette and no wall. This means that outside wall has no support tie to the camper floor any more.
But the bed frame is still there tie'ing the floor to the wall. That helps.
The question comes down to running length of the camper with no left to right support of the wall to the floor. And there is no support on the opposite wall which runs from the front of the camper to the sink.
If you look at all the other camper floor plans, there is no camper that has that long of no wall support to the floor on both sides at the same time if you pull the entire wallout.
If you left a cabinet where the wall was, or only cut the wall down to 2 feet long or something, then you still have a support tie from floor to wall.
My "thoughts" are the camper "might" flex left to right a lot more if you clear span both sides like that. Meaning the entire camper structure might "parallelogram" so to speak after a lot of towing. I myself would not clear span both sides like that that long.
Now that couch, is that a jack knife coach? If so it mounts on a wood frame to get it up off the floor. If you build a long frame to tie the floor to the wall and put the couch on top, that brings strength back to the trailer side flex.
All the cabinets and mini walls they put in the camper creates stiffness left to right.
A point to think through, removing the wall will (may) create holes in the ceiling and the floor tile. You will have to patch them up. The patch may or may not blend in well. It depends on how creative your wood working/ masking skills are to create the patch.
The fresh tank, can you take a few pics and post? I do not know your camper layout to see what you are up against. It might be easier then thought or as difficult.
The plastic the fresh tanks are made out of have a real hard time having adhesives stick to them. So the tank manufactures heat melt or mold fittings to the tank.
Help us see your tank and the fitting area. We might be able to help short of the taking the "entire camper"
Hope this helps