Good grief.
The LEO says he knows his regs so why lecture and all the needless blather about regs.
Agree with the custom fabricated hitch welded into the trailers main frame and cross members.
I've owned three 5th wheel trailers and had custom fabed hitch on all of them for double towing.
I wouldn't mess with a bolt on unit.
RV doubles (two trailers) around here are very common and have been since the late '60's when RV world developed and became popular.
Some big rig trailer repair shops can do this type of work. Check with them.