The six-passenger full size Chevy has a luggage compartment with a folding cover at the rear by the tailgate. That's were I put my 8 gallon RCI tank. The compartment where the tank sits needs to be covered as well as vented to the outside, and I secured it with a toggle lock. It's just sheet metal back there, no cross-members, so I may modify it when I have some down time!
Good luck!
Lew