Rather than carrying it inside the vehicle, you could consider a hitch mounted cargo carrier. There are many styles out there, but, they generally are about 2 ft by 5 feet, carry 500lbs, and they are supported by a standard 2" trailer hitch. Most SUV's have a 2" hitch standard now, along with trucks obviously. We use it to carry things like a generator and gas cans. (And the gas grill for football tailgating.) You want to make sure you have a heavy duty one, because some are only for light use. Still, they keep everything outside the passenger cabin.

For the bug out scenario with some theft prone folks, you might have an issue with the outboard storage...but, they do have steel locking cargo boxes too... or you could see one of the threads on firearms for society breaking down type scenarios <img src="/images/graemlins/wink.gif" alt="" />