Ken. I would totally concur with your thought process.
Weight is the key consideration here...not length. Given the same type of construction materials one has to think that this may well add possibly 10-15% or more weight to the total trailer weight depending up on configuration. An extra 300+ lbs could possibly put him over on his RAWR as well as his GVWR...or not. I only know that weight was my first consideration in seeking our first 5th wheel.
I have a SRW 2500HD CCSB truck. 9900 GVWR which is quite low. It took the wife and I near 3 months of looking to find a suitable trailer that we liked and that met my requirements as far as weight is concerned.
To be honest, I am still over on my GVWR particularly with the truck loaded/w full load of fuel (Titan 52gal + 30 Gal slip), wife/dog - that puts me at ~8200 lbs. Leaving 800 for the pin. No way one can find a newer model (even an "ULTRA LIGHT"), with that kind of pin weight. We ended up with the trailer in my sig at a stated pin weight of 1490lbs...and we use it - A LOT.! Added a Roadmaster Active Suspension kit for added stability and to eliminate some minor rear sag.
I'm good with it. May not be 100% within Manufacturer spec..but its as safe and then some as many more out there. (I also happen to believe that GM rated this particualr truck low on GVWR)
The other thing to consider with additonal length is side load...WIND. That's one heck of a long rig - hit a cross wind..?? and it could be a bit dicey no.??
Anyways Guys, just some thoughts/comments on a slow afternoon from the Alberta Oil Sands. And BTW eh.!: Get that Keystone Pipeline going..!!! (-:
cheers
Stk