Overheating while towing at highway speeds

I recently bought a 1993 E150 conversion van with a 5.8L engine. I bought it from a doctor friend of mine who used it to go on trips and to tow his boat. He did tell me when I bought it that it would occasionally overheat while towing, but that he'd done some work on it to correct that--he had a new radiator recently installed, plus a new heater core and a new thermostat. Most of the coolant hoses are also new. The van is a Starcraft conversion with a heavy duty AC compressor, TV/DVD, interior lighting, etc. so there's a pretty good power load on it.

I bought the van to pull my camper, which is a 1994 28 foot travel trailer. I'm not sure the total weight of the trailer, but it's a good size and being a mid-90s model it's heavier than the newer ones but not as heavy as the really old ones. I've pulled that trailer with a number of vehicles (until recently I owned a car lot so had a choice of vehicles to take on trips) and never had a problem pulling it. Generally I pull it with a full-size SUV (Tahoe, Escalade, Suburban, Expedition, etc.) but I figured this van would handle it fine.

Anyways, last week I hooked it up and hit the road. Within about an hour of driving 65-70 mph through moderate hills the van started heating up. I watched the temp gauge even as I turned off the AC, TV, lights, etc. I finally pulled over once it got near the red zone, and the coolant was just starting to boil over. We grabbed a bite and waited for it to cool down. I bought some coolant but only added about a third of a gallon and it was back up to the cold fill line. We restarted and I kept the speed down to about 60 mph. The temp stayed dead center of normal, and I was able to run with AC and everything without anymore problems. We ran another 3 hours or so without a problem. We stayed a couple days there visiting family and I had a local mechanic flush the coolant system and check it for leaks. He found a loose hose clamp that we assumed to be the problem. Next day we hooked back up and continued on our way. But... another hour of driving (same conditions: 65-70 mph and moderate hills) and once again the temp gauge starts rising. We go through the same routine--cool down, little bit of coolant, then restart. Once again we hold our speed down to about 60 mph and it runs normal. A couple times the temp started to climb when we hit some bigger hills but the first time we were due for a stop anyways and the second time was just before we hit our destination so I'm not sure if it would have leveled off or overheated again. On the way back we took some back roads (maintained speed at about 55 - 60 mph with some start/stop through some small towns and some moderate hills) but the temp held steady the whole way. Since then I've also run the van several hours without the trailer and never had a problem cruising the whole way at about 75 mph.

Any ideas you have would be helpful. My one thought is that it could be the water pump--I'm not sure if he had that replaced or not. Have you heard of water pumps acting this way? Can you buy a heavier duty water pump to handle these conditions? Anything else anyone can suggest?

Thanks.

might need an external tranny cooler, maybe an external electric fan.

Did he install a larger radiator for towing or just a stock radiator. A good radiator shop could tell you by looking. These radiators have more passes.
For what you are doing ask he will either install the correct unit or sell it to you. Otherwise you can cause engine damage. I would also check my fan clutch don't buy a silicon unit get a centrifugal clutch unit. You might find a fan blade with extra blades at a junk yard. Make sure you belt has proper tension.

After a number of years, the vanes in the water pump can wear and not circulate enough coolant. Check the fins in the radiator to make sure they aren't clogged with debri/bugs, restricting the flow of air through the radiator.