Les
We did all the research, as I have said we used the Trip Advisor forums and all the recommended information sources suggested on there....and we came to the conclusion that it was cheaper and more practical to use motels and a rental car. This is especially so if you are exploring both islands and covering a lot of mileage, as a car is easier to drive and more economical. Also your car rental company will usually collect the car from you at the inter island ferry terminal and supply another one when you get to the other side - thereby saving you the cost of taking the car on the ferry. With a motor home you generally take it on the ferry at your cost.
Regarding motels, we found they were superb, and offered full kitchens so you could make your own meals if you wished, and had a lot of home comforts - some had washing machines, dishwashers, etc, and TV and radio. In fact some of them we would have been happy to live in!! They are excellent value and cheaper than B&Bs (which are generally up market compared with the UK)and hotels. Both of the latter don't normally have kitchens.
Motels are located in towns and outside - practically everywhere you would want them. Find them on web sites such as Expedia - just put in the locations and dates and they will give you all the information, pictures and user reviews. There are companies such as Travelbug.co.nz who will also come up with deals - although you can sometimes beat their prices by doing it yourself.
A lot of people would advise you not to book if outside of the Christmas period, through to the end of January. But I feel that if you know your route it is better to do the research, look at reviews and then you generally know you have booked a good one!
One other comment - not all will agree, but we found that the cabins on campsites were nowhere near as good as the independent motels. Oh..and there are motel chains as well where if you choose a good one you can be sure that they will all be the same standard.
Hope this has given you something to go on. You will certainly have a great time. Enjoy the planning - its much better than going with a travel firm (although you might like to use a flight agent - or at least compare their prices).
Keith