I've got quite a bit of gear I stuff into my camper for long trips. I tend to have pretty much everything I need and then some. I carry enough tools, spares and fluids to choke a horse. Camp gear, food, water and other beverages. Since my camper is open to the cab of my truck much like yours will be it is ultra important that everything is inside a cabinet or the lower bench. If it's not in either of those it's tied down some how. Mainly because I don't want loose stuff hitting me in the back of my head if I hit the brakes hard avoiding something on the road or coming down a steep trail.
I've learned the hard way to that aspect. Taking mine off road also proved that if it's not tied down it's going to look like a bear tore then place up when we stop for the day. Everything goes all over the place.
I've got two large open cabinets with doors on my drivers side, each have a plastic tote that carries different items. One holds dry foods/breads/snacks. One contains camp gear. Each cabinet has a little room to stuff other items in or around the totes and the doors are lashed to each other with a rubber latch found on some roto-molded coolers. The third cabinet nearest the door isn't as deep as the propane tank enclosure fills part of it's depth. But I leave smaller items like a 1/2 used roll of paper towels (a full one won't fit), bungee cords, flashlight, remote for my LED lighting inside the camper (being able to turn some of the lighting on and off from outside before climbing in is great when it's dark out). There's probably a couple other items in there I can't remember now. But it's handy because I can reach into this one just standing outside the camper with the door open.
My large bench on the passenger side is where I stuff my extra bottled water, gatorade and beer that I don't need to have in my fridge as I rotate them in as I use the ones in the fridge. Also contained inside is a soft gear bag of larger camping gear, lantern, hammock, straps, air pump for inflating mattresses and such. Another soft gear bag has my spare fluids, 2 gallons of coolant, brake fluid, power steering fluid, ATF and engine oil. There's a couple spare lockout hubs for the front axle, serpentine belt, zip ties, tire repair plugs and mechanics wire. in the corner I've got a milk crate with an extension cord, jumper cables, a couple chunks of 2x4's, fuel pressure gauge and a spare stub axle shaft. Next to that is a tool bag with my large tools, 1/2" drive sockets, ratchet, breaker bar, large pliers, BFH, and two bags containing my metric and standard end wrenches. I'll have complete spare front axle shafts stashed in there before my next trip too. There are some other spares stuffed into the corner for the engine, ignition wires and injectors.
My camp stove is tied down to the top of the cabinet and it's hose is stashed in the propane box outside. Fishing poles ride up there too as the length reaches into the upper bunk area along the wall out of the way.
Underneath the cabinet on the driver side near the door is my main tool bag (atlas 46 tool roll) that's easy to reach if needed. A folding shovel is also right there in easy reach. I've got a roll up style aluminum table that also stows under the cabinet, but it sits on top of the wheel tub in a tight spot with a folded up tarp.
My kitchen gear is loaded into one roller style pelican case that fits snugly between my ARB fridge and the lower bench. Near the door I stack in my chairs and the step needed to get in and out of the camper easy.
The only remaining original storage space in mine from FWC is the little narrow cabinet with slider doors above the window on the passenger side. I keep my inflatable sleeping pads in there along with a couple of rolls of TP and a stash of plastic grocery sacks.
I'm probably overpacked for what I do, but I follow the idea of "better have it and not need it than need it and not have it.".