IIRC when the older local doctor left Walton's mountain because he couldn't make a living there due to people not paying him in actual money the residents of the area made a pretty big deal about finding a replacement which of course is what brought Dr. Curt Willard to the area and the show. But a couple of years later Curt's was call to active duty by the Army and never returned until of course his death at Pearl Harbor.
When Curt came he and the locals also made a big deal about raising the money to buy a bunch of new medical equipment and fixing up a local clinic to far better serve the medical needs of the community.
So who took up the role of doctor in the Walton's Mountain area after Curt left? I would assume that Mary Ellen did eventually as she did become a doctor but I think that was a few years in the future.