Do you plan to have a vehicle to drive around once you get to the location and set up? If so, the stretched tractor may be better and just tow a vehicle behind. With the trailer you are stuck using the tractor to drive around, unless you build the trailer to haul a vehicle.
You can fit much longer than a 20 ft living quarters box on the stretch if you want. Of course depends if you are keeping the sleeper or going with more of day cab and then LQ behind, but easily could be 30-35 ft of LQ with day cab style.
Most here are longer wheelbase and have the LQ with the tractor, or the std truck conversion motorhome. However, there are many folks that are starting to use a class 6 or 8 tractor to pull a large 5th wheel trailer. This would be analogous to your trailer type build. Some of then put a small car (Smart car for example) on the tractor behind the cab, and in front of the trailer hitch. Most of these 5th wheel tractors are single rear axle, since they do not come close to the weight loading requiring two axles.
2005 Kenworth Showhauler 45 ft total length
twin screw, 450 hp ISX, 10 speed Ultrashift