I believe the first show geared 100% to truckers was the Road Gang on WWL in New Orleans starting in 1971 and hosted by Charlie Douglas and Dave Nemo.We can't forget the Truckin' Bozo (Dale Summers) on 700 WLW and several other stations, including WHAM, WWVA and KWKH at various times. When Dale had to retire due to medical issues, his son, who had been doing the weekend versions, took over, the show was dropped by most affiliates, though was re-named The American Trucker Radio Network. Earlier, there was Interstate 7.
While Bill Mack started in 1969 on WBAP and WSM had an overnight country show since the early 1950s, these programs also tried to appeal to other people who were awake overnight. Truck drivers were the only listeners the Road Gang catered to.