ah right, yep, add a leasure battery, the biggest he can get, think mine is 105ah, split charge system, and a 240v inverter.
have a look on the xbox and it will tell you the power supply wattage, i would just get an inverter a bit bigger than this as they get very expensive as you get bigger. do a bit of reading up on inverters as there is a fair bit too them, like most are modified sine wave, the pure sine wave ones are even more expensive.