1. Outsourcing - Using external services, e.g. Showers at your local public or private swimming pool or public beach facilities. Cost: Free, or up to $4.00. Of course you can live hand to mouth, shopping daily at Countdown or New World, so you don't need to refrigerate perishables. NZ is a fairly cool climate anyway.
2. Fuel cells: There is a new gas-fired fuel cell available in the USA which will greatly improve energy use by campervans because fuel cells offer up to 50% electrical efficiency, thus reducing the amount of heat loss.
3. Solar energy can be an attractive solution for larger campervans, though its not the best solution given its seasonal limitations and still low conversion efficiency. These are not such an attractive option, as batteries are also expensive.
4. Mains power: The other source of power is mains supply. This makes sense if you are taking short trips, but for long distance travelling it really does not make a lot of sense. You can use the mains supplies from campervan parks but you will pay a premium for it. This option only makes sense if you are going to stay there anyway, say for the security.
Expect in the next 2 decades some new combustion engines to be developed which will offer greater fuel efficiency. There is a lot of money going into research these days. The US Dept of Energy is throwing money at technology companies to develop new products.
Andrew Sheldon www.sheldonthinks.com