I purchased a cargo carrier from Tractor Supply Company (TSC) several years ago. It held up well, in all weather. I used it on 2 different travel trailers. It was designed to slid into a single receiver. BUT! It had a lot of "slop" (as stated above).
I ended up making a frame under it with supports that went and rested "under" my rear bumper. I had legit receivers on both trailers, welded to the frame, factory.
Once I added those extra "supports" that were designed only to keep the thing from rocking back and forth, it worked very, very well. The problem was, with all that lumber I had to add, it made the carrier pretty heavy. So, the only thing I ended up carrying on it was a plastic simulated steamer trunk I used for my sewer hoses and my garden hoses for water hook up. When we had to break camp in rain, it came in real handy to lay the yard mat on (rolled up of course), so it could drip, or get rained on more, and not make the inside of the trailer compartments wet.
Then our current fifth wheel also has a factory receiver hitch and the carrier fits perfectly fine. But the problem is, if "anything" is on the carrier, it blocks the rear compartment storage door, and that is where I keep all my landing gear, boards, electric wires, chocks, everything for initially setting up the camper.
With the carrier there, I'd have to remove everything off the carrier, then remove the carrier to access all that stuff first, before even remotely setting up (like blocks under the tires and such). Nope, with the current fifth wheel, it was just too much hassle. It now sits in a shed a home and hasn't been used in 5 years now.
That is going to be your biggest challenge .... getting the severe rocking, back and forth under control.
If you are installing a hitch, get a double receiver, one one each side and get a carrier that has 2 slide in's.
Like this one: