Chrono251:
Thanks for the reply.
My understanding is that the MOSFET has to go on the positive side, to avoid leaving the electronics "hot". That's why I was considering a P channel. At home I have some IRF540, that are N-channel power MOSFETs.
'Hot" is only relevant if you've got other devices around you which are grounded and there's a danger of touching both with something conductive. A 12V battery on a hill probably isn't much risk.
Either way will work, but with a P-type you'll need to be able to switch the gate of the MOSFET between GND and +12V, which the Arduino can't do directly. You'll have to add a pullup resistor to 12V on the gate of the MOSFET ("off") and another smaller transistor between MOSFET gate and ground for the Arduino to be able to switch the MOSFET on.