A lot depends on the weight of the trailer when finished and loaded with all the toys and camping paraphernalia you will haul. You will need to know gross overall weight, and pin weight (the amount of weight pushing down on the hitch). RH hitches have about a 8 inch dia. bearing surface (about 50 sq in) while Tractors have about 24 inches dia. (about 450 sq in) or 9 times the surface are to support weight. most semi trailers tend to have the rear wheels much farther back than on a RV trailer, this is done to increase the amount of legal weight that can be carried over bridges etc, by spreading the weight out. this results in a much higher percentage of the overall weight being placed onto the pin. As RVs are much lighter and do not have to worry about the bridge law, they tend to move the axles forward to both reduce the pin weight on truck, and to improve turning radius. Using a semi trailer which is longer than most RV and heavier interior construction methods, I think you are going to have far more pin weight than any RV hitch is rated for. More weight on a smaller bearing surface will also cause far more wear. I believe that the biggest hitch in RV style is 30,000 gross weight and 7,500 pin weight. If you can get within these limits it may work. Calculating finished weight of a project can be difficult. you will need to know the empty weight (and weight at the wheels and pin) of the trailer you will be using, then add in weights of building materials that are evenly spaced along the trailer (wall finishes, flooring etc) and then point loads of appliances and fixtures with their center of gravity. then you can assign a percentage of the weights to the wheel and pin based on their location. This will give you an educated guess as to what it will weigh. (its easy to forget how much fastener, little bits of steel etc add up. If your state will let you keep the commercial 5th wheel most of this becomes a moot point as you are unlikely to overload anything with that set up. (some states do hit you with a larger registration fee if you go over a specified weight limit) and make sure you can get someone to insure it. lots of threads on here about that...
There a lot of companies making mid-weight cargo trailers with either RV style hitch or goose-necks. A bit more expensive then a used van trailer, but still far more sturdy than an RV.
Definitely not trying to be discouraging, just want you to know to do plenty of research and have your ducks in a reasonably straight row before you create something you can't license, insure or isn't safe. Read thru the many threads on this site, there are several folks who have done a great job of documenting their builds and issues they have had. This group is a great bunch of folks and arguably the best resource you will find on this stuff.
Dave
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