The fan in my 1999 Winnebago Chieftain would do the same thing, as well as shriek sometimes. Removing the cover and spraying some lube into the motor would free it up and work for a time. Did this many times over the last 2 years. It finally froze up and wouldn't restart. Copied the Manufacturer name and model # off the ID plate on the motor. Found a new one on Amazon. Replaced it. Now it runs like a champ. Should have done it 2 years ago.
BTW, I had Camping World check it on my maiden voyage. They estimated about $1200. Turned them down. The motor I needed was only $179 on Amazon.