From what I understand our '72 Safari at 3500 pounds dry has a tongue weight of 500-600 pounds, well over the 7-10 percent rule. The axles are pretty far back almost like a boat trailer. To carry a spare tire and bike without adding to this I had receiver hitch and tire stand fabricated for the rear bumper. I've read all the risks in doing this and talked to all my engineering friends and have no worries. The 1 1/4" receiver crosses a 2" cross member between the frame rails and Tee's into a shorter cross member bolted to the trailer frame. The tire carrier bolts to the end of the trailer frame.
We arranged it where the tire sits just below the tail lights and is just higher than the bottom the bumper.
The bike rack just clears the spare tire.
on a side note, we added racks to carry kayaks on top. Far enough forward to clear the camper in tight turns.