Apply for a passport if you don't already have one. American citizens don't require a visa to visit New Zealand as long as they don't plan to stay more than 90 days.
Take out travel insurance. Check various companies and policies, as there are some questionable travel insurance schemes. Ask friends for recommendations.
Get an International Driving Permit. Although New Zealand may recognize the driver's license from your state, an International Driving Permit allows you to drive in New Zealand, ensures that you don't have any trouble getting caravan insurance, and serves as an additional piece of identification.
Study a map of New Zealand and decide where you want to land. If you want to start in the big city, arrive in Auckland. If you want countryside tranquility, head for Wellington or Christchurch.
Do your research. People in New Zealand drive on the opposite side of the road (the left side) and say "g'day mate" instead of "hello." They also use British terms, so be familiar with the caravan's windscreen, boot and bonnet (windshield, trunk and hood).
Plan your route. Decide where you want to start and where you want to end up. You might, for example, want to land in Auckland, rent a caravan, tour around the country, and then leave your rental and fly out from Christchurch.
Decide if you're going to rent a caravan online or if you'll wait until you arrive and then check out the options. Either way, be sure to buy caravan insurance, as many of the roads in New Zealand are narrow and winding and there's a high rate of accidents.