Originally Posted by BASE729
I know a 30 lb propane bottle is 24" in height, but that can go in the crew cab while in transit to get filled (or on its side if allowed).
I would not carry portable cylinders on their sides, full or empty. Upright only.
You could get a 20
-pound cylinder as a spare, that should fit in the space available.
Just about anything you carry in the truck can be laid on its side, except containers of liquids and your generator— which is also a container of liquid in the form of its gas tank— that will need to be upright to prevent spillage.