My opinion is that plastic sweats less than steel because sweating is caused by something remaining cooler than the temperature of the environmental air. Plastic being lighter can follow easier as it takes fewer BTU's to make the temp change.
What is your fuel source? Is it on farm storage or delivered from where? Do you fill from a tank in the back of your p/u? Do you have a water separating fuel filter on your supply line like Goldenrod co. offers? I'd suspect the source as where the water originates.
Additionally, when you run your tractor, do you let it come up to normal operating temperatures, or do you just make short runs especially in the winter?