First thing is to NOT go out and buy a lot of stuff. You probably have most anything you need already in your home, so maybe try what you have before you buy. One of the biggest mistakes many people do is to go out and buy, then figure out later how much they really don't need or use.
You will need a good certified safe water hose, sewer hoses and such, a few varied extension cords, and for me, a portable took kit with wrenches pliers, and whatever in a case that stays in the RV. A good charcoal or gas grill, tablecolths for picnic tables. Several various flashlights, extra batteries of all sizes. And MOST IMPORTANT, a Sense of Humor, and the wisdom of when it is needed.