Your profile shows you are from IL. If you intend to tow such a combination in IL then it would not be a legal tow in IL. The only vehicles that can be towed behind a 5th wheel is a boat, ATV, PWC, or motorcycle. Also the trailer would have to have brakes.
Here's the applicable section of the statute.
625 ILCS 5/15-107(c)
(5) Recreational vehicles consisting of 3 vehicles, provided the following:
A) The total overall dimension does not exceed 60 feet.
B) The towing vehicle is a properly registered vehicle capable of towing another vehicle using a fifth-wheel type assembly.
C) The second vehicle in the combination of vehicles is a recreational vehicle that is towed by a fifth-wheel assembly. This vehicle must be properly registered and must be equipped with brakes, regardless of weight.
D) The third vehicle must be the lightest of the 3 vehicles and be a trailer or semitrailer designed or used for transporting a boat, all-terrain vehicle, personal watercraft, or motorcycle.
E) The towed vehicles may be only for the use of the operator of the towing vehicle.
F) All vehicles must be properly equipped with operating brakes and safety equipment required by this Code, except the additional brake requirement in subdivision (C) of this subparagraph (5).