I've gone two seasons with the FWC on my K5 and learned a ton the first season.
My FWC is unique that it bolts onto the body of the truck just the way the stock fiberglass top did. Well, it does now. Some got bolted to the front of the cab like the factory top and I've found some didn't. Let's just say 5 bolts down the bedsides don't quite cut it. So I added a small section of the factory fiberglass top as a header to the front wall of the camper and bolted it to the cab also. Needless to say mine don't move and I don't have turnbuckles or straps to deal with. I do check the bolts to make sure they are tight every now and then. Haven't found one loose yet.
Now the stuff inside is where I had a steep learning curve on. When we refurbished the interior the cabinet doors used standard friction locks similar to what you would use on a cabinet door at home to hold it shut. Woefully inadequate for anything off road. The first big trip I took the camper on was the Mojave road. That's a lot of washboard. Doors flopping open all the time. As the suspension flexed and shifted weight side to side they would fly open too. Pretty quickly I figured a way to lash the doors shut with bungee cords I had stashed in my camping gear. Which meant we had to undo at camp and set back up prior to leaving. Annoying.
The fix was simple. I bought a package of rubber T-handle latches off of Amazon. They are very similar to the ones found on some rotomolded high end coolers. They have a catch that screws on separately, then the handle screws on. Space them out so there is a little bit of tension on the T-handle when latched and it stays nice and tight off road.
I rebuilt the lower bench to include a hinged lid with a fold out flap for a wider sleeping surface this year before the wheeling season started. Problem was the lid and flap rattled like a lumber wagon on a paved road. I could only imagine the noise off road. The rubber T-handle latches came into play again. Along with using the little felt adheasive backed pads where the sections met for a little softness to touch before two sections came together.
The bench with the latches.
In this current form I've hit up 3 mountain trails here in Colorado this summer and the Rimrocker trail from Montrose to Moab with La Sal Pass tossed in, Hell's Revenge and Flat Iron Mesa trails in Moab. Rock solid. No doors rattling. I have an ARB fridge that is strapped to the floor with D-rings. The cook box (a good sized Pelican case) wedges into the space between the fridge and the bench. Everything else is located inside the cabinet compartments or inside the bench.
A buddy was with me on the Rimrocker and Moab trails with his 77 K5 with a similar FWC camper. I had warned him about tying everything down in the camper or it's going to end up on the floor once we got off road. He thought he did. We got through a good chunk of Hell's Revenge when we stopped to watch the tube buggies climb out of one of the hot tubs. It was a good time to eat lunch we he came back to tell me it looked like a bear got back there and tore it up! Everything that wasn't tied down was on the floor. But as I reminded him that's still a problem since our campers are open to the cab of the truck. If something bad was to happen and you stopped quick (or stood on the nose as we came close to out there) that stuff is going to end up on you in the front seat.
Needless to say he took more time tying stuff down for the next trail day.