You will need to check you state requirement as well as the GCWR rating of the truck. Some states will allow some things to be towed behind a 5er (must be a 5th wheel hitch, not a goose neck) and they do place some length restrictions on the rig as well as some require a special endorsement on the drivers license.
next problem you will encounter is even if you are legal to tow a second load on the back of you 5er in your home state, some of the states you cross will not allow this.
Another issue is can the TH frame take the additional stress of say an added 4000# to tug along? Better check with the manufacturter on this, not the dealer.
Also, noted this is your first post, and welcome to IRV2.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot