I am lucky enough to have four doors and a long bed. Here is how I transport my Big Rig (13'3"):
I have a full bed box that raises the bed floor 10", plus a futon on top (a nice cushion for my yaks to rest on!), so I welded up an adapter for my bike carrier:
I attach the midships grab handles to lashing points in the bed, attach a lashing strap through the closest scupper holes and then through the kayak adapter (also a place to run a Python Lock), and lash the window down, as well. I have driven with this set up just over 200 miles at 65mph, and not even a shimmy! The front 1/4 of the yak is supported, as well as the support of the yak adapter at about 1/4 from the stern. There is no contact on the top rail of the tailgate, to prevent creasing the keel. The bike rack pivots down to allow loading through the lowered tailgate. The only draw back is dust inside the camper, when I drive to certain locations with unimproved roads.
cricman