How big is the trailer?
How hot was it while towing?
What driving conditions (lots of steep grades, stopped in traffic, etc...)?
Some things to look at before getting exotic check some simple items...
- Does the electric fan come one once up to temp in stopped conditions?
- Is the radiator full?
- Pull the thermostat and test.
- Check the radiator fins to make sure they aren't damaged.
Assuming the trailer doesn't exceed the truck ratings, a bigger radiator shouldn't be needed and I've not heard of upsizing a radiator before.