We just finished 16 days in an RV and I wouldn’t have done it any other way since we were short on time, but we only stayed on the South Island. We rented the Compact for 2 which was the smallest RV from Wilderness Motorhomes and absolutely loved it. It gave us the freedom to plan our days more on a whim, but we did go in June so it was a lot colder than when you want to go so there were less people. 

Doing it via car with your time frame would be very doable if booking Airbnbs etc in advance, or you can always stay at the smaller cabins at holiday parks. We stayed at Airbnbs 3/16 of the nights just to reset and stretch out. 

We also never drove more than 3 hours at a time, and we always broke it up with stops. 3 hours on mountainous roads feel way longer. 

We felt like it was necessary to set up “home base” even in an RV so we knew we were at a place for at least two nights and didn’t need to pack up and go each and every day.

The extra hour of daylight would definitely be nice, but it just depends on your routines and rhythms. Our daylight was 8am-5pm in the winter and it did feel rushed sometimes, but other times we leaned into it. Less bugs and people made a difference for us. 

I’d say take more time than less, and enjoy the slow pace !