2 kinds frequently were available for the bus, similar in some
respects but not in the important ones.  Both typically had an 1/4"
thick
L shaped plate that bolted to the face of the bumper in the center.
Through this went a steel bar tongue, solid 2" x 3/4" or so, now
on the cheaper this went back and sat under the rear engine mount bar
around which there was a U bolt that positioned it, this plus being
bolted to the bottom of the L that projected from the bumper and
had a couple of holes for safety chains was what you got.  Unfortunately
didn't work well and reallly killed rear engine mounts.  On the better
ones the tounge went back to a framework that hung between the
two frame attachment points for the rear engine mount bar.  Both
were typically only class 1, we purchased one of the better sort
from Whitney in the late 70's when our '76 was new and towed
a 6 person pull camper for many summers with it with no large problem.
If this is what Whitney still sells I don't know.  Cost $65 or so at the
time.  Later I picked up a very nice heavy duty class 2 hitch made out
of a heavy famework of 1.5" square steel tube that bolts directly
to the frame under the bumber mounting bolts.  Found it on a junker
bus.  I would look for something built like it, make a frame that goes
between the rear frame rails.  And be prepared to really slow down on
steeper grades, take it easy and don't overwork and overheat the
engine.

John