I had a local welding shop install a receiver on the back end of my fifth wheel RV trailer. They bolted and welded the receiver frame to the frame of the 5er. I had Big Tex build me a 5'x10' box trailer with 4' tall sideboards and one 5,200-pound axle with ST225/75R15E tires. That gave me a hitch weight of about 200 pounds and payload capacity of about 2,000 pounds. We hauled a quad plus big generator, or a golf cart plus the generator, then later a handicap battery-powered scooter plus the generator. Texas Motor Speedway first outlawed the quad, then they made it almost impossible to use the golf cart, so DW wound up using her electric scooter to get about a half mile from the primitive no-hookups campground to our seats in the grandstand.
We drug that trailer behind the 5er for 10 years and more than twenty 600-mile round trips before we stopped the expensive hobby of attending the NASCAR races at Texas Motor Speedway.
The Good Sam/Trailer Life campground guide includes a summary of towing laws for the USA and Canada that shows where "triple towing" is allowed. They define triple towing as one tow vehicle pulling two trailers. It's allowed in 37 states and provinces, but usually with caveats such as the first trailer must be a 5er, not a bumper pull. Pay attention to the FedEx rigs in your are. If you see a tractor pulling two trailers, the first will be a 5er and the second will be a "pup". If you see that on your highways, then you know that triple towing is allowed in that area.
Caveat: that second trailer is going to do a huge sway every time the 5er does a little bobble. Not a good. My cure was a sway bar on the hitch of the box trailer. Like this one:
Note that you need a special shank/ball mount that includes a provision for adding the tiny ball for one end of he sway bar. Reese says "◦Can also be used with ball mounts that have sway-control platforms, even when no weight distribution is used." Here is a ball mount with provisions to add one sway bar. They also make them with provisions to add a sway bar on each side of he trailer tongue.