I agree that the majority of us (rigs) are probably overweight. Not sure about other states but triple towing behind a 5th wheel is legal in CA as long as the total length of all vehicles doesn't exceed 65'. But you might need a Class A license.
And, although I haven't done it myself, one of these days I plan on adding a slide in receiver/cargo carrier to the back of my 5th wheel for fire wood, gas cans etc. and maybe even one of the kid's quads. Check out RV.net in the toy hauler category as others have done this mod successfully.