Depends on the model of radio you have....., but with my 2018 Jay Flight there are three "Speaker Zone" buttons marked 1, 2, and 3 located in the upper right corner of the radio controls (iRV Technologies radio).
Bob
__________________
2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)