I agree 100%. I did the same mistake with a International 4900...
the International I had was about that, 34 feet long and towing about 4000 to 5000 lbs trailer. In California it was kinda ok. keeping it below 60 mph gave me a not too bad milage and you could still talk in the cab ... running 65mph was loud, 69mph was rev limiter at 2600 rpms.
towing in the flats was ok, hills 35mph up a 6% grade loaded light. the 11 mile grade out of laughlin was endless going up heavy. It took me one time 30 minutes
and the international had a high torque (675 lbsft) DT466, not very usual in Penske trucks (but the truck was also red not yellow
how much torque has the 8.1l ?