I agree with many of the items that people have listed, with a few exceptions.
-Unless you are staying hooked up for long periods of time, a sewer hose support slinky is, IMO, a waste of money and more trouble than it's worth. I bought one, used it exactly once, threw it away.
-In 35 years, I've never needed an extension cord for my RV, although I will admit that our friends did indeed need one last year. Once.
-A 50A/30A dogbone (as needed for your RV).
-Toilet chemicals of your choice, and remember to use some in your grey tank as well; it can smell worse than the black. If you want to try the "water only" or GEO method, more power to you, but I'm happy with my chemicals (I use Happy Campers).
-Two 10' sewer hoses at minimum (I have a 3rd that I use at home to reach my sewer cleanout, but typically only carry 2 with me), with a clear elbow (at both ends, preferably).
-Get a 50' water hose and cut it into a 15' and a 35'; add hose ends. You can use either the 15', or the 35', or the full 50' combined as needed without pulling out any more than needed. I've found that 15' is sufficient most of the time.
-Water pressure regulator.
-A camping mat helps to keep dirt/debris out of the RV.
-Wheel chocks and leveling blocks.
-Basic tool kit and multi-meter.
As others have said, keep things simple the first few times out; you will figure out what you need.