NM my brother is creating a better post in regards to this.

OG

 

Below the seat.

 

If you're putting a mattress back there, build a false floor with a slide out drawer accessible from the tailgate.

 

this was my first thought. I've seen many pictures of this kind of thing for tool storage. If you want to make it fairly universal you could make the drawer the size of a rifle case that will fit what you plan to put in it. Build the drawer to fit the case and you can just drop the case in without having to add padding/means of securing the rifle inside the drawer itself. That way the drawer would be useful for anything else when the case isnt in there.

Sleeper cab conversion sounds interesting. I would enjoy more details about the plan and some pictures if you have them.