After leaving Four Star, Aaron Spelling joined forces with Danny Thomas to start their own production company. The union lasted just 3 years, with Thomas walking away. Spelling, of course, veered away from sitcoms by then. In fact, I think this might've been his only one with Thomas.
Rango, as a mid-season replacement, wanted to be a complement to F-Troop, another sitcom set in the Old West. Instead of an army base, however, Rango (Tim Conway, ex-McHale's Navy) was a Texas Ranger. He was a bumbling, pratfalling sort, not unlike F-Troop's commanding officer, Wilton Parmenter (Ken Berry), forever exasperating his commanding officer (Norman Alden). In short, series creators Harvey Bullock & R. S. Allen tried to cross F-Troop with McHale's Navy, albeit with fewer elements from either show, plus modeling Rango after Maxwell Smart (Get Smart), and failed. Director Sidney Lanfield worked with Conway on McHale, so there was some continuity after a fashion.
Unfortunately, Conway would discover time and again that while he was a valuable commodity as part of an ensemble, he couldn't carry a show by himself. I don't think Rango even finished out its only season, and it would be Conway's only project working with Spelling & Thomas.
Gilmore Box provides the intro:
Viewers voted with their remotes. With Get Smart in its 3rd season, they didn't want a clone, even if Conway were to end up with a sexy sidekick of his own (but didn't).