You might try a slide-out from a wrecked RV or trailer....just be aware you may need hydraulics to actuate it, and they are notorious for leaks and hoses exploding. Lessee, hydraulic pump, hoses, tank, expensive fluid, electrical system....
And a tendency to slide out while driving on the highway, according to some horror stories I've heard....RV catalogs sell rods meant to hold the slideouts CLOSED while driving, so the stories must be true.
And being RV components, they are probably made VERY light-weight, so I'd question their durability and strength.
You would also need to reinforce the bus where the cut-out is, to provide stiffness and strength that you are removing when you cut the wall out.
I've seen a couple major motor coach mods on-line (can't remember where, obviously) that used welded steel frames, actuated with rack-and-pinion setups driven by electic motors and gear reducers. Nice thing about this is they used a manual crank to retract the slideout, it engaged the reduction gear motor shaft through a hole in the floor. Took a HELL of a lot of cranking, but what the heck, if it worked for a B-17 main gear, it'll work for you....
RV catalogs also sell SUPPORTS to put under the slideouts when parked and extended, seems the frames tend to sag after a while.....
The tool storage is nice, but where do I put the bed?