I can't speak for the IKSS floorplan but on both my old 233S and my current 235S I took the que from Rockwood and found one of their false panel covers that were inside cabinets and unscrewed them to see how long the screws were that they used, and simply used the same length.
I added some panels to the front pass through of my 235S by making a triangle wood stick frame that screwed into the floor and then the inside cabinet frame and never screwed into the exterior wall. I believe you can see that in this picture here... the right stick is screwed to the cabinet and I have the leg going at an angle to the left meeting the horizontal stick that makes my triangle... and THAT flat stick on the floor has a single screw through it into the floor;
One final thought... on my rig I have easy access to the Rear Tail lights and market lights, and the hole that Rockwood drilled for the wires to come through allows you an easy view of the cross section of an exterior wall. I think if you can find something like that you might be pleased with actually how thick those walls are.
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Ed & Chris, Empty Nesters (missing Josh & Jaclyn)
along with Blue & Tessa, Cat Explorers!
2020 Rockwood Roo 235s
2019 F-250 Diesel 4x4 Super Duty