I have been on dozens of trips and I always have something new to buy but the basics will get you started. Then the things that Camping World wants you to think you need - I like to set up parameters around that.
Definitely need to start out:
* Sewer hose with attachment for different systems.
* Fresh water hose.
* Non-potable hose for dumping (rarely needed in my experience).
* I keep a soap holder and bar of soap in the same compartments for washing my hands. I don't use that garbage pump stuff some people like - I want to wash my hands not swirl the dirt and germs around.
* Blue stuff. The regular walmart stuff is great. Or "treat" yourself with Porta Pak lavenders. Delightful.
* Regular old toilet paper is fine - Scott is fine.
* Good house battery (hopefully that comes with your rig).
* Lots of lighters both regular Bic type and the long ones.
* Flashlights - with good quality cheap LED flashlights available - why not have 2 or 3 - I even have a headlamp now.
* Duct tape - when I didn't have it I needed it - when I did have it - I didn't need it. Get some.
* Emergency Roadside Service.
* Spares of different bulbs used in the rig will eventually be needed.
* Journal
One of the most useful items I have is a journal. I use the front part to log each trip almost as a diary. Then I use the back part in reverse to write things down I need. Excellent memento and very helpful for keeping organized.
But the parameters for things I need:
* Avoid one trick pony types of items, such as stove toasters, etc. I prefer anything I bring on board to multi-task when possible.
* Rule of 3 - if I want something 3 times I probably will give in. Space is limited so I don't want to bring everything but the kitchen sink.