Wow... that's a very broad question. lol Some people's "gotta have" may not get used by other folks. Having said that, I start and I'm sure lots of others will chime in....
Fresh water hose (white), I bought 50' and cut it into 10', 15', & 25' sections so you always have the right length; water pressure regulator; in-line water filter; sewer drain hose; disposable gloves; first aid kit; fire extinguisher(s); emergency tool kit (even if you don't know how to use it, someone who stops to help might); lots of flash lights (they get lost easily, especially with children); rainy day games or dvds; a notepad and pencil to write down things that you want for your next trip; a laptop or tablet to access this site for questions; lots of patience and good sense of humor (things won't always turn out the way you think they will); and last but not least... duct tape, it fixes everything in an emergency.
Lots will depend on what you buy and how you intend to use it. Lost of people start out spending a night in their drive way or somewhere really close so that if they need something that they've forgotten they can just run inside and grab it. Good luck.
Doug