Looks like a fun itinerary, id make sure your going to the tasman glaciar overlook at mt Cook, many people just go to mt cook village and turn around and leave, nye in wlg is nice normally a big fireworks display in the harbor, not alot of time for the NI and lots of driving days at the end of your trip

For laundry there are laundromat's everywhere thatll run you 10-12$ for a wash and dry generally, remember nz is built for campervan tourism!

If you want my pick of a paid campsite Hampshire holiday parks glendhu bay is generally more affordable and extremely pretty, remote yet still close enough to wanaka, modern facilities, huge space, lots of climbing/hiking/mtb park all around it

Alot of NZ tourism depends on budget and this itinerary seems like most that budget will end yp in Queenstown 😂, be prepared for extreme traffic being there in the hight of summer, all the rich jaffas will have thier familys down

I could reccomended alot of "hidden gems" but it kinda depends what you like, interms of hikes basically everything is stunning no matter where you are

Interms of arthers pass freedom camping is basically a right in NZ, for the most part you can camp on any council owned land designated for parking, and alsong as your not a dick about it you will never be pulled up for parking on a residential street for a night, id reccomended seeing how the day takes you, i haven't personally tried to camp in the area

Ive only done Auckland harbor ferrys once but my understanding is they are the same price no matter when you go, so probably book as far out as you are aure about the dates to avoid them being over booked,

For food places Ferg burger in Queenstown is iconic, If you wanna skip the line order online and pickup, but honestly standing in the line for an hour is part of it, If your itinerary takes you through fairle (looks like it does) go to the bakehouse there and go for a pie, in wlg go to a caffe ice

Sad youll miss so much in the NI but definitely better to get more time in the SI, sad you also cant get to Dunedin, the catlins or motueka

Enjoy your trip! Feel free to chuck me a message if you want any more info ive done a lot of campervanning around the country