Make sure they include a mounted spare tire... I was amazed when a friend bought a TT and had to pay extra for the spare. I always assumed a TT comes with a spare! Spare tire cover is nice, and you can paint it if it has an obnoxious decal / advertisement on it.
The TT should come with a ~25ft power cord and sewer hose. As others posted, you will probably end up buying extra lengths of power cord - I just bought mine from Walmart, so I have 25' and 30' cords - works well when you are just a few feet shy of a power outlet, or if the power outlet is on the "wrong" side of your campsite. The sewer hose will work for awhile, just buy a replacement and keep it in the camper until you need it. Ask if the dealer will include sewer hose adapters, including a 45 degree elbow (clear if possible).
Awning "deflappers" - clamps with velcro that attach to the awning and braces to help prevent the awning from being whipped around by the wind. Get a pair!
Depending on the dealer and how large of a store they have, ask if they'll give you some sort of discount card or credit for their store. As you get camping trips under your belt, you will figure out what extra stuff you want.
Try camping a few times close to home, so that you can return to get stuff that you find you need. Try not to buy too much stuff right away - most everyone does, though, and you end up with various things that you find you really don't use.
Everyone does things a little different, using different products. Take all this advice, and figure out what works for you.
