Building what TomC is would be my perfect outcome but I have to do everything myself as this is the only way I will ever afford it. I figured the bus frame would be a close counterpart to stretching a standard semi except without the headache of stretch the driveline and frame....
As a poor ol' country boy who started with nothing and still has most of it left I can help here! Stretching a frame is not hard nor expensive! Unless you want to be hard headed and picky!
We used to build HEAVY DUTY WRECKERS! And I mean HEAVY DUTY, so they had to be done right to keep them from folding in half!
What ya do is you get an idea of how long ya want the frame to be stretched! OK now keep in mind here your building a toy hauler here not a wrecker so we can do several things here that will make it easier, more convenient now & later, save you time, weight, headaches now & later, and most importantly $ now and later!
OK first thing ya gotta do is get the desired truck! All right your building a toy hauler and camper out of it, so you don't need a twin screw! (single axle will work fine!) Once you have the truck and you have an idea how long ya want it, then ya go to the truck wrecking yards and start looking at their driveshaft piles. Trust me if it's much of a wrecking yard they will have one to dig thru. OK now find a drive line about the length of your desired stretch. Now it doesn't have to be exact because your not building it for NASA so it's not rocket science that you can build the hauler to fit the new length of the truck! OK let me stop here and back up a minute. On the length of the frame it doesn't have to stop right at the end of the axles like a wrecker does because your not going to be lifting or hauling huge loads. So here is a way to save on how far to stretch it. Also if you buy a twin screw take the front drive axle out and sell or trade it for other things you'll need, then you can move the rear drive axle forward to the front ones mounting holes (or you could drill new ones which can be done but extra time, & hassle!).
Now when you have the drive line of choice gathered up, you use Jack stands to support the front half of the truck in front of where your going to cut it! And the drives if tandem will support the back. If you bought a single axle use a HEAVY duty floor jack to support the back part. Then use a plasma cutter, or sawzall, or torch and cut away! (I like plasma or sawzall's for clean cuts!)
Then once the cut is made you bolt the drive line together, level the frame and measure the gap. Once ya know the gap go back to a wrecking yard and get a section of frame off a similar truck the right length. Once you have the new frame pieces you support it in place and tack weld it. then cut some flat plates (we call 'em fish plates) to weld on it on the inside of the frame. Now weld the fish plates on where half is in front and half is behind the weld. Then come back outside the frame and fill the gaps which welds the frame and the fish plates at the same time. The rest is easy stuff like splicing air lines and wiring!