Wheel chocks are a must, you can get them at Walmart for 1/2 the price as your dealer.
I would buy a good quality sewer hose, the one that will come with your starter kit will not be pleasant to deal with. I bought mine at walmart, although I think they have a different style now. Mine is orange and brown, and I love it.
Rope lights are really nice to have, they help put light in the places your porch light doesn't hit too well.
ant poison, wasp spray and mosquito repellent, to leave in your camper. You will be amazed at how many camp grounds I go to and have to fight off ants at the picnic table, or when I open the power box that there is a wasps nest!
water hose pressure regulator is a must.
Every summer I buy a box of cheap-O' flash lights from walmart. They are those $1 led lights that come with the batteries already in them. A hole box will set you back about $14 and it's great for the kids as well.