The usual function of that switch is to connect the rear a/c to either the generator's secondary output or to the 30A shore power. The generator always feeds the front a/c through the 30A main supply, so the generator position allows both front and rear to run together when the generator runs. When switched to the main 30A supply, it normally disconnects the front a/c and replaces it with the rear.
If you have the powerline EMS, it may be allowing both front and rear to run on 30A shore power at the same time, but this can only be done if other loads are off or have been shed by the EMS. 30A isn't enough for both airs and, for example, the water heater on electric.
I don't see how both a/c could run on shore power with the switch in the Generator position. Maybe a former owner modified something? Hopefully another Sightseer owner can comment on how theirs works. I took the liberty of modifying the subject line to include "2006 Sightseer" in hopes of attracting one of them.
