You have no hope of Wellington - Queenstown - Auckland in 8 days, unless you do almost nothing but drive (say 75%). If the 8 days includes day of arrival and day of departure even less realistic. The average driving speed in NZ will be a lot lower than USA.

Many camper van rental companies, with vehicles of varying age & quality. Do not go for the cheapest (oldest roughest). Google is your friend.
--> https://www.vroomvroomvroom.com.au/r...n-new-zealand/
---> https://www.drivenow.com.au/campervan-hire-new-zealand
and other web sites. (these are not recommendations)
Juicy are at the low end of the camper market. Seem to have small vehicles. And plastered with Juicy signs, so the open invitation "we are visitors - please steal from me".

There is likely to be a one way drop off fee. Many people go north to south, so there is usually a surplus of rentals available at southern depots.
This web site (and others like it) offer cheap one way rental most south to north:---> https://www.transfercar.co.nz/ (119 in Queenstown now to go north. And 29 in Wellington for Auckland).
Some rentals do not allow you to take cars/campers across Cook Strait, but drop off in Wellington and pick anther up in Picton (or the opposite)

The Cook Strait ferry takes ~3:30 hrs, plus time to board etc. Depending on the departure time, say the best part of 1 day for that, unless a very early or very late sailing. North to south is better viewing IMHO. The cost of camper on the ferry is ~$250 - $350 or more one way. Bookings are recommended, but they are good at putting you on an earlier sailing if space is available, if you line up in the standby queue.

With 8 days just visit 1 island. If flying into WLG and out of AKL suggest the North Island only. Leave the South Island for another time when you have at lest 10-14 full days to explore.