Adding switched power to AC Fan
 

Long and short, I have dual batteries and would like to add a switch to the AC fan. Ideally this is how I would like it to function:
In 1 position, i would like the Ac fan to function normally when my AC is turned on using my starting battery.
Position 2, disconnects it from the AC circuit and switches the power source to my Auxillary battery. In a dream world, this position is illuminated.
What switch do i need? DPDT? DPST? 3 TERM? 6 TERM?
THANKS