Check the placards on all the components you will be using at the same time, add up the number of watts they will draw and get an inverter with enough capacity to cover the total.
If you just want to run an entertainment center, I found a 750W unit bought at Costco adequate to run a 30" LCD flat screen, Sat receiver, Blue Ray and small sound system. I mounted the inverter close to the batteries and ran a cord to an outlet I installed in the entertainment center. I took the end plate off the inverter and soldered a couple of wires to the on/off switch terminals, then ran them to a switch in the entertainment center so I can turn the unit on and off from inside. Works great.
One 12V won't cut it though. We get away with twin 6V's but rarely run the inverter for more than two hours before re charging for a couple of hours. Again, you can use the total load you came up with and your projected usage to come up with your battery needs.
