On my old Mirada, I have the same thing. The A/C and heater systems are actually from Evans and Coachman installed them. (I'm not sure about Berkshire)
When I select VENTS, the A/C compressor comes on and air blows through the vents. When I select A/C, same thing.
To resolves this, I added an A/C switch so I can override the compressor regardless of the MODE selected on the controls. The MODE must be in any position other than OFF,
This mod does not override the pressure or freeze switches.
AC switch controls AC compressor regardless of MODE switch | 2001 Coachmen Mirada 300QB