I just did trial and error to find a good location, marked it, drilled holes and bolted it in. I put a pad of rubber between the compressor feet and the fender to help reduce the vibration noise.
For the switch I used one of the blanks above the fuse panel on the dash. There is a grommet already going through the firewall that I used a coat hanger and fished the wires through.
It is looking like I may have to add a new access port though for my CB coax and other wires.
I don't have dual batteries in mine. I have Sears Platinum Marine 1160 CCA battery that should be sufficient to run my accessories. I use a power distribution block from the battery and then to a aftermarket fuse block.
Small things like the compressor get power from the aftermarket fuse block. Large things like my amp get power from the distribution block. Both are fused.