You may already know this....
The screws holding the interior panels are of the PoziDriv type, #2 size. Using a philips head will result in some additional work.
(HomeDespot or bLowes)
The interior wall panels fold under the windows, then rivet to the exterior steel & top rub rail.
Cut the steel about an inch below the windows. I used an Ocilating tool & radius blade (pictured). A grinder, cutting wheel or other means will do well, too.
My IC RE300 is similar to your FE300. Please peruse my Demo albulm, I've written details in the photo descriptions. You may them helpful or at least entertaining.