I do believe this one has been done, but my reference escapes me.  Nonetheless, the project is so simple it's easier to do it than find the missing reference!

We start with a standard 30' CMW van and use the side panels to scribe the sides and the top with a razor saw.  The top is luckily the same ten feet as the side panels, offset, so we have natural joining lines to blend in the cut lines.

To cut the floor, I first tried reversing the sections whereas this puts the joint offset form the joint that runs through the side.  This did not provide as much overlook as I wanted, so I ultimately flipped the trailer floor and made one cut immediately ahead of the rib that butts up against the axle springs, and then a second immediately behind the seventh rib, or ahead of the valley of the sixth rib, removing seven valleys and six ribs in all.  Cutting from the bottom has the benefit of the ribs providing very straight cutting guides.

Test fit...Everything fits pretty snug, though I could have been a little more precise with my cuts if I had more patience.  Eh.

The end result is this short 20' Pup Van.  When I checked the fit of the gear, I discovered the old holes are now too far back, so I will eventually have to redrill these holes and make a cut in the under frame web so the gear are in their proper place.  Otherwise, It's a trite issue.  The second trailer awaits - for another night!!