I know on my All Terrain Camper Shell, built by the guys that used to work at FWC, both the drivers side and the passenger side are the same... with a 1"x2" horizontal aluminum square tube that the fixed part of the slide out bed attaches to.
Here's a photo of the drivers side.
On the passenger side that 1"x2" square tube is right above the window opening. Sorry, no photo of my passenger side, classified details over there.
The ATC interior side walls are normally held in place with square drive screws at the top and the ends (they staple the bottoms, however, I asked for screws all around so I could completely remove the interion side walls and the insulation, too, in this photo).
If your FWC has those square drive screws at the top you can remove them and peel the interior side wall back a bit to take a peek and some measurements.
Or, maybe FWC's are completley different.
Just thought I'd take a shot at helping.