Overheating engine: 1991 K1500 Cheyenne 4.3L 4*4

I have been having a problem with my engine overheating
while pulling a trailer! I am unsure if it will do the same
thing without a trailer. I do not want to drive it until the
problem is resolved! The other day, I was pulling an empty
trailer to another city, about an hour and a half away. It was
fine for about 10 minutes when it started this. The normal
operating temperature is about 160, however it was doing
about 180, almost 190°F. I was driving between 45 and 55
mph. The faster I went, the hotter it got. It would drop in
temperature when I had to slow down for a while, while
driving in smaller cities where the speed limit was 35
passing through. I know of a few things that wouldn't be a
problem like the radiator and heater core. I think that it
might be the thermostat, but it could also be the water
pump. I want to know what else could cause the engine to
overheat when going faster speeds over long distances.
Thank You!