Dealer hopefully will give you a starter kit. Use the cheap sewer hose until you figure out what works for you. People will tell you to buy a rhino sewer hose. Frankly I have seen more of them in the dumpster than any other brand. What ever you do, stick with a specific system. Get a multi fit elbow, connectors to join more than one hose and a decent hose support.
As for potable water hoses. Start with a 25 foot, maybe two. Get the 5/8 hose for better flow.
Electrical. Depending on the trailer, 30A, get a heavy duty extension cord and a few adapters.
Back on water, get a brass "Y" and a good brass regulator. You will never regret this.
Leveling. I like and use 2x6 boards cut about 24 inches. Cheap, durable, and if you happen to drive off and leave them, no big deal. I also have an assortment of 4x4s to use under the stabilizers.
Thats a good start.