Could've/should've been funnier, but 'The Apple Dumpling Gang' is still an entertaining enough 100 minutes.
I had a solidly good time with it, it never drags and has a few decent characters and a watchable plot to follow along. As noted, I think they were going for something way more amusing, especially with Theodore (Don Knotts) and Amos (Tim Conway) - who I liked watching, but they aren't overly funny.
Bill Bixby (Donovan) and Susan Clark (Dusty) team up to strong effect, while Harry Morgan probably gives the best performance as McCoy. The villains are largely forgettable, though Slim Pickens is OK as Stillwell.
Not much more to add in truth, except for acknowledging the nice set design. All in all, the film isn't anything incredible but I'd still very much recommend it.