Well, the must haves include one or more water hoses (I have multiple lengths, but that's a choice not a requirement), one or more sewer hoses with a drain "L" connector (usually comes as a set, again, I have several of varying lengths).
Beyond that, you may want a couple of AC adaptors to ensure you can attach whatever size your rig has to whatever the campground has. I'm a 50A rig, so I have adapters to get me connected to 30 and 20 amps (with associated loss of power capabilities). A ladder is handy. Some carry a short one to get to the side of the rig and access the rig's built in ladder to get to the roof, or you can get collapsable extension ladders that get higher access, but don't stand alone. You can also buy a wastewater tote if you are going to be camping where you'll need to dump elsewhere. I've got one, used it only once.
After that you'll find yourself adding a second set of everything from tools to dishes, kind of like having a vacation home. Then if you go to any camping store you'll find that it's full of specialized goodies that you just can't live without - you'll think.
Tony