The summer months from December to March are by far the most popular with visitors to New Zealand - not just those who rent a motorhome. Not surprisingly, in summer the crowds are the heaviest and you can expect to pay top dollar for most items including airfares, motorhome rentals, campsites and attractions. From about December 26 until January 10, most New Zealanders take their annual vacation so the popular seaside destinations will be overrun with throngs of Kiwis dressed in shorts and jandals!
If you visit during the shoulder seasons (October, November, and April), you'll avoid the crowds, feel a lot less like a tourist, and your motorhome rental will cost you less, and your holiday cash will go further which means you can stay longer. South Island weather is usually no better or worse than in summer and North Island weather is mild and normally fine (though that can never be guaranteed - it's the rain that makes the grass so green!)
If you want a really inexpensive motorhome rental holiday, tour New Zealand in the winter (between May and September). Because demand for motorhome rentals is lower, off-season rates are drastically lower. But beware, you could be very cold as most rental motorhomes are built for three season touring. Make sure the motorhome rental you choose for winter touring is insulated and has carpet on the floor as well as a diesel or gas heater that you can operate when you’re not hooked up to mains power. A rental motorhome with a drying room for wet towels and clothes is a real bonus in the winter.
Tip: You can save up to 75% of the cost of motorhome rental by travelling off-season. But make sure your rental motorhome can cope with the cooler weather.