Under 250 hp, and under 400 ft-lbs of torque, and you are asking it to move 20k+ down the road. Diesels work well, but its not like they are magic. It should work, just don't expect to drive fast.
What rear axle gears do you have?
I have talked to a few guys running old 24ft uhauls with Navistar 444s (non turbo 7.3s), hauling one in the box and one on the trailer, and they can go 60-63 mph or so, slow on hills. (4.56 gears, spicer 5 speeds, etc).
Tell your friend to buy you some goodies from banks if he wants you do tow him around.