Here's Simrad's blurb on locating the RC 42.  

Select a location that provides a solid
mounting place free from vibration, and as
close to the vessel's centre of roll and pitch
as possible, i.e. close to the water line. It
should be as far as possible from disturbing
magnetic interference e.g. engines (min. 2
meters), ignition cables, other large metal
objects and particularly the autopilot drive
unit. On steel hull boats it should be
mounted 0.75-1 m above the wheel house
on a non-magnetic stand.

I realize you may well have read that and be looking for some real-world experience.  I have my Raymarine EV-1 sensor core about 1 meter forward of the rest of my electronics, which is inline with Raymarine's reccomendation.  Previous to this autopilot I had a fluxgate compass installed under the floor in my master stateroom.  I ended up having to move the compass because of interference from a source I struggled to identify.  My advice would be to try the location you're considering and see if you find issues. 

Ben

Thanks Ben.