Driving down the road, the length of a long bed crew cab is not an issue. But getting one in and out of my driveway, and especially maneuvering in tight quarters, makes the combination too long. Although my driveway is "circular" in that it has two entrances, there is a tight 90 degree turn in the middle with an ancient Alligator Juniper right on the inside of the turn, and several grown Chaparral on the outside. I don't think I could get a long bed CC configuration around that turn without backing and filling at least twice, and I would not be able to maneuver the trailer in the driveway at all for lack of turning ability. (Maybe I need to build a dolly for the trailer to get it parked in the yard, and put a 2" receiver hitch on the front of the CJ-5 to run the dolly.)
Add to that the fact that my wife would never in her lifetime drive something as long as a long bed crew cab truck, and the use of the truck would become limited to pulling the trailer. The wife is already after me to get rid of one or more of our 5 vehicles, and to get another truck I need to sell off at least one and perhaps two of the trucks we already have. If I do that, I need the new truck to be versatile enough that we can both use it. That's the thinking behind the question.
I understand the problem with compound extensions. Whether the trailer frame would take the extra stress, I don't know. If it will, the extra extension would serve a purpose at low cost. If not, converting back to 5th wheel and using a slider might be the only option. Still, it is hard to choose without knowing the options. I was looking at the pin box extension yesterday, and wondering if an additional extension would cause a problem. I would be more concerned about how the pin box extension is attached to the frame and the shear load on the attach bolts. The frame of the trailer in this area is seriously stout. One of the local RV businesses might be able to say whether the combination would work or not, and if the frame needs some fab work to resist additional stress, I have those tools.