There are battery isolator systems available that let you charge both batteries off the vehicle alternator, but isolate the vehicle battery from all but normal vehicle draw, so it's always available for cranking. I'd use a deep cycle on the second battery position for better longevity.

Trail fix:
Camping with old friends and got whacko and played the cd all night, then crashed about 4am, and left the interior light on. I was the last one to leave, and it was dead as stone. Had an 18v Dewalt tool battery with about half a charge on the recip saw we'd used to cut up branches for the fire. Took the voltmeter and figured out + and - on the battery, and used about 12" of Romex scrap to hook it to the truck battery. Cranked first try...