94 1/2 B250 5.9, 3 speed auto, long wheel base.
28" travel trailer 7000 lbs (32 ft total length)
I have had failure in as little as 40 miles when travelling at highway speeds. If I stay off the highway, and keep it below 55 mph, I can get 100-120 miles before it starts to act up.

 

You might check the Cat converter. I had something similar happen to my truck but it turned out that the Cat converter had come apart inside and would block exhaust flow at different times. Trucks don't run to well with no exhaust!

Tar

 

You could try premium gas when you go towing. Although it is likely something inside the engine is giving up, if you are only getting a short distance.