....the fire extinguisher should come standard with a camper from the manufacturer. If you buy used, I'd recommend discarding the fire extinguisher immediately, and buying a good quality refillable/serviceable model, and if you're sleeping waaaay up in the cabover, have the extinguisher right on the kitchen counter top beside you, where you can get it in an emergency (both for fires extinguishing & personal defense).
Also, if your cabover does not have easy egress (for emergency exit), keep a cutting device (like a carpet cutter) n the cabover at ALL times (slice through the vinyl/canvas to escape an emergency).
You have a good basic list of items. However, I'd recommend you create categories of emergencies and camping, like this:
-lighting & emergency visibility;
-emergency foods;
-vehicle emergency tools (truck tire repair; jack; ratchets; etc);
-emergency camper materials (pop-up wall patch kit; hose repair; etc);
-personal safety materials (1st aid kit; PLB; cellphone & backup; etc)
....then, add your missing items to those categories; then, store those category items in a place that makes sense to retrieve said. If you start without categories, you'll get all turned around, and you'll be missing potentially life-saving items...
On edit:
Beer: if you drink beer, make sure to store some of the highest quality brands you can afford (like Old Speckled Hen: Tasmanian galaxy hops; Bells Hopslam; Alchemist heady topper;etc) :B