I second the suggestion to put a 25-amp fuse in the slot, and see if it makes a difference. It can't hurt anything.
Also, though, try to get to the fan housing (it should be relatively easy to access under the hood), and see if you can free up the motor. I had a similar issue on a past coach (F53 chassis). I removed the fan/motor assembly from the housing, and it ran fine at all speeds. When installed, though, I found that the fan was slightly off-center, and was binding in its housing. I added a couple of washers on one side when replacing it to straighten it in the housing, and never had another problem. I doubt that will be your situation, but once you get that far, it's easy to replace the fan motor.
Mike and Trish
2014 Thor Tuscany 40RX
2016 Jeep Grand Cherokee on Blue Ox Aventa LX
Yorkiepoos - Janis and Jimi