It sounds like the sofa you are describing is very similar to mine (and norty’s) tho my 5th is a ‘13. In my case the wall behind the long section was finished. The half-wall was only partially finished, the part that was visible.
We never liked the thing to sit on and our son n DIL complained abt sleeping on it.
It’s now gone. Removal was pretty easy. IIRC it was not screwed to the floor at all but the “L” piece was screwed to the half-wall which seemed to keep everything in place. I opened the bed, removed the air mattress and started removing bolts. The seat backs were held in place with spring clips and removable once I got it away from the wall. As you suspect the “L” piece is basically a wood box. I was prepared to use a sawzall to make small pieces out of it but at the end of the day it came apart in small enough pieces to be handled by 2 people and went out the door, once out it went to the curb. I’ve replaced it with a 3 recliner Thomas Payne theater seating sofa. I left it free floating for about a year and the current project is to secure it to the slide floor.
The pix are the bare wall, the paneling I installed and the new sofa (minus the seat backs)
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Cheers,
T_
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks