Adding second deep cycle battery

I've been thinking about this and I would really like to add an accessory battery. Like a smaller deep cycle in the trunk. This way I can add a 120v outlet (I'm building my fourdoor for camping). Anybody else done this? Id want it to charge off of the alternator of course.

Just look at RV wiring diagrams. Usually you put an isolation relay, diodes or switch, in the circuit as 2 dissimilar batteries in parallel will try to equalize. With different capacitance/internal resistance they tend to destroy each other unless very equally matched when not being charged.