Originally Posted by 4RunAmok

If only there was a short, easy, concise answer to give.

The upside/downside is that while you're driving to camp, the vehicle's alternator is charging the second battery. The downside to that is that your vehicle alternator is designed to keep your systems running whilst pouring it's excess into your starting battery keeping it charged. Rarely will a stock alternator do an excellent when you add extra batteries. Better systems will obviously add a lot of cost to do it right (DC to DC chargers, proper solar setups, etc). The simplest shortest explanation here is this: You will likely never charge your auxiliary battery to the top, which eventually will begin to damage it the more you use it, and you'll be replacing the auxiliary battery much earlier than you expected.

Even upgrading your primary battery to a Group 31 like I have, you will find, as I have, that a daily commute is not enough to keep a beast of a battery like that charged. I never have trouble starting, but I find my battery rests throughout the week between 11.9 to 12.2 volts (an AGM should rest around 13 volts fully charged, essentially meaning at 12.0 volts, my battery storage capacity is at 0, still plenty of energy to start my 4Runner, but if I tried to run my fridge overnight, it probably wouldn't make it through the night) Even after driving many hours, I find that a stock alternator will not charge the battery to full capacity.