condensation ?

[Rawze]

If there is excess moisture from coolant getting in, it will be its most concentrated at the very bottom of the oil pan.

At next oil change ..

Clean the drain plug very well, then wipe it down a couple times with clean engine oil. Pull the drain plug and let it go into a cup right away. Send that to a lab and ask them to look specifically for coolant intrusion.

===

You could also put a coolant pressure test kit on the coolant system then drive the truck under heavy torque load and see if pressure is building in the coolant system too rapidly.