I'm going to bump this back up, I'm coming home in a few weeks and need to get my 39 foot quad slide fifthwheel and the 4runner home. I am currently planning on flat towing the runner behind the fifthwheel. I am dealing with a lot of the same issues that are currently being discussed here (length weight and tow rig GAWR and GCWR) my truck is an 07 duramax 3/4 ton sbcc. I have towed an enclosed behind it with my chopper in it to Sturgis and it was no problem at all, although I assume the runner will be around 3k heavier than the motorcycle trailer. The truck deals with the fifthwheel fine, I'm slightly nervous with the flat tow to be honest, 4 states and 1200 miles in the northern US in December seems like it could add a fair amount of unknowns. I did just add a set of airbags to the truck to keep things level and smooth.
Paul, I know you said you have done this (kinda) anything you can remember that was an issue?