You're likely to get a good reply and list here, but I'm to lazy to make you a list, and no longer have mine.
But...I made my list by looking at Youtube videos about trailering. Not only do you learn a lot about what you'll be doing in terms of towing, setting up camp, and so on, you'll learn a lot about what items you'll need. We were very successful using this approach. We ended up with a mountain of stuff from Amazon and a couple other sources, and I can say that nothing we got was inappropriate or the wrong thing.
Poke around, do an internet search, etc., and you'll probably find a couple of lists, or a forum thread on the same subject.
There were a few items we had on our list that we weren't sure if we'd really need, but we've slowly added them as we've discovered that yes, we need them.