I understand, and there may likely be some "standard" knob that you can buy at the hardware store.
But I don't get the impression that RV Doctor George would be wildly expensive. His website sure is low key. Makes it look like their prices would be reasonable.
I called them when I was looking for furnace
parts. They didn't know if they had what I needed but took my number. An hour later they called back to say that they did not have what I was looking for, but they sure were prompt and pleasant. They also gave me a number for another place to try.