Jamie,
If you want to cut the constant power that supplies your main switch panel when you shut off the car you could wire in a continuous 12V on relay inline to your 75 amp panel feed circuit. Available on e-bay cheap/ they are solenoids typically used on electric golf carts We use higher amperage ones to isolate the 2nd battery on stockers that run one battery in the trunk and one up front. Not sure I would to count on one to supply everything, I'd probably put in a big circuit breaker for short protection to my main panel.
http://www.ebay.com/itm/NEW-CONTINUO...hT4lZr&vxp=mtr