Water! Get a decent 5 litre water container. First night out I had to wash my cooking stuff up in window cleaner and brush my teeth in coke zero. Tastes foul.
To be honest all you really need is water and wet wipes. Food and entertainment can be found away from the cab.
Also, don’t park on a downhill slope. You spend all night rolling out of the bunk 
(i’m in class 2 without the luxury of leveling suspension)
Edit … and preferably a duvet instead of sleeping bag. And check your inverter can handle the wattage of your laptop … Yes my first night out was ■■■■■