I can tell you that the double truss extensions from Torklift have their current rating due to the extension strength and their receivers have more capacity. If you are building your own extension, it will need to be comparable in strength to get similar ratings. I have not seen any other receivers with side plates as large as those used in the SuperHitch, so I question whether these other receivers can take the leverage a long extension exerts on it with any significant weight.
My 20' enclosed trailer is typically loaded at 6000-8000 lbs and has an 800-1000 lb tongue weight. I cannot see a 7500 lb trailer with 500 lb tongue weight unless it is loaded rear heavy or it is a boat.
