This episode was recently shown on MeTV and I watched it a couple of times. The "Notes" section below discusses the idea that Floyd reveals that Uncle Joe was the engineer of the Cannonball prior to Charley and Floyd, but I can't confirm that from my viewing.
When a replacement for Floyd is first discussed early in the episode, Joe offers his experience of running the Cannonball several years earlier when Floyd was sick (and, as Steve notes, running it into the Pixley station). He doesn't say anything about having run it years before (and running it better then), which he certainly would do in keeping with his character if that was the case.
At the very end of the episode, Floyd decides to un-retire, and he talks about all the letters he received asking him to come back and in the last line spoken in the episode, he says Joe "was the first one to write!" With his drawl though, at first listen it does sound like he says "he was the first one to run it!" (and on the basis of the Notes section, I watched the whole episode waiting for someone to say something like that). But "write" makes more sense in the context of the dialogue, and after listening again I'm sure that what he says.
Uncle Joe being a retired engineer is an interesting idea, since we don't know much about his backstory to begin with (which side of Kate's family is he from? Does he have any ownership claim on the hotel?), but there really isn't the evidence here to support it.
While this episode does make a reasonable series finale, it's too bad they couldn't bring back Homer Bedloe and Norman Curtis to bring the story of the Cannonball full circle.