Thanks everyone for all the info, it helps.   Steve - yes, I'll have to cut crossmember(s) at the hitch location. Might even need to open the bed completely to the rear but I'm hoping not to. I've got tentative plans either way. There's also some crossmembers missing in the pics that I had to remove because of the axle locations so I could get the bed temporarily mounted. (M1078 is single axle, my Freightliner as you can see is tandem). Had to travel over 1,000 miles to get the truck here. I was able to bring and borrow just enough tools to get the bed modified enough to fit and safely stay on the truck. I'll trim those crossmembers for tire clearance and relocate/reinstall them after I get the frame extensions and hitch done.

Apologies, Rod. I bumped it once, then lost track of it for you. I did see the latest video, but didn't put the two together. GLWS.

Okay, I'm wrapping my head around it now.  I applaud your re-purposing an existing bed to save money.  Sometimes, it's easier to just open your wallet, but not nearly so satisfying.

You need to account for swing and dip clearance.    Dip maybe not so much with a camper?     The bed shown has a crossmember about where a hitch may be located?     As far as height above the deck of the bed to the nose of the trailer,  6-8" should be OK.    I have 5.5" with my Kentucky van using a Holland commercial hitch.   With a 36" pin the dip will rub on steep turning events very lightly.     I have outdoor carpet on the deck and it shows mild abraison.      My deck is about 4" high off the truck frame      With my Trailer Saver TLSB mounted on 5/8 stock on top of the truck chassis the hitch height aired up is 51.5"    The bed is at 44" so the vertical is 7.5" and I have miles of clearance fo the camper.      

Thanks