When we were over we stayed at a campsite in hut (lower i think) just outside welly and it was clean and tidy with a good tv room (important as the super 14s was on )
As we were out of season we got a good deal on the camper (kea campers)and there did not seem alot of difference between motels or the camper.
so it rearly depends on when you are going,and if peak season you may find that you need to give a bit of notice to book a place at motels or campsites(ie phone ahead)
But we did stay on the side of the road a couple of times just because we could or needed to sleep right then, the camper will give you greater flexability but the insurance (optional ) can be steep.
we were with out the camper for the first 10 to 12 days of our trip and came unstuck in new plymouth no vacancy at any hotels, motels holiday inn(sorry just felt a rap comming on)
Because of a big rugby match (hurricains v the cheifs) but a lady in a motel rang her friend and a dear old lady put us up for the night ,God bless the kiwi helpfullness and trust
All the best
Johnty