It sounds good. I'm a kiwi so I can probably answer general questions about NZ. I left twenty years ago although I still have family there and keep a close watch on the news etc and listen to NZ talk radio while lying in bed before I get up most mornings but anything specific I'm probably a little out of date on.
Hopefully travel will have opened up by 2023. Right now I think there's a mandatory two week quarantine in a designated hotel which you can't leave when you arrive and several months waiting list for a spot so I couldn't even easily get in as a NZ citizen right now.
Credit cards generally work the same way as they do here. Visa and Mastercard are widely accepted. I had no problem with my US card in NZ in 2004 (last time I was there). I would still check with your bank or someone who really knows because I vaguely remember hearing about possible problems if your card doesn't have a PIN. NZ was heading towards almost being cashless when I left so I suspect it's closer now.
I had a look at the site.
https://www.fantasyrvtours.com/RV-To...NP-032022/rl/0 It looks great but I think there will be some long days driving. 100 miles or 163 km on NZ's mostly two lane roads is a lot more work than 100 miles on a US interstate. Day 19 from Waitomo to Orewa sounds like a long way especially since it goes through Auckland although I guess you can stay on the motorway (freeway) through Auckland.
As an aside, that day includes two places of significance to me. You pass Hobbinton which is in the town of Matamata where I was born and my elderly parents still live and then Orewa is where I last lived and owned a house until I moved to the US.
The roads are pretty good quality but away from the main cities they are generally two lanes with intermittent passing lanes. Of course you'll need to get used to driving on the left.
I don't really know about cellular access. I think it's pretty good although data limits are probably much more expensive than here. You can usually roam with your US phone but I know you need to check it out carefully first or it can be very expensive. An alternative is to buy a local SIM or even a cheap local phone when you get there.
I'm sure you've weighed up the options but an alternative of course is just to hire an RV and do your own thing on your own time. I think that's a popular option. I have some friends here who did that a few years ago.
I'm sure Australia will be good too. The physical countryside is definitely different and the culture is similar but a little different. It's kind of like the difference between US and Canada.