dodge 3500 cummins clutch fan engages on flat with no load

Sounds like a thermostat.

It could be a clogged radiator, but that doesn't sound like a long enough drive to really even need much of the radiator.

It doesn't sound like an air bubble either, they run hotter than that. I think you mentioned it, but you have verified the coolant level right?

You "should" see it clime to 195-197°, drop to 185-188° and then slowly climb back up to 190-193° and stay there. A hill might make it hit 197°, but it should take a decent load and hill to break 207°.