Once a NZ motorhome trip stretches past 21 days, the rental economics change (weekly rates drop meaningfully past day 21 on most fleets), and so does the route. You can fit in Stewart Island's three-day Rakiura Track, the Catlins between Invercargill and Dunedin, the East Cape between Gisborne and Opotiki, and the Far North above Kerikeri without compressing the main loop. Self-contained certification (a fixed toilet + grey-water capacity, NZ Standard 5465) becomes worth the slightly higher rental cost because it unlocks free DOC and council freedom-camping sites that 2-berth conversions can't legally use.
Where the extra weeks go
From a 14-day baseline, the most-recommended additions in order are: the Catlins and Bluff (adds 3 nights from Dunedin); Stewart Island walking (3-4 nights including ferry from Bluff); the East Cape coastal road (4 nights between Gisborne and Whakatane); the Far North above Kerikeri (3 nights from the Bay of Islands); and Mount Aspiring / Glenorchy on the Routeburn side (2 nights from Queenstown).
Self-contained vs. holiday-park nightly
A self-contained motorhome can legally stay overnight at most DOC campsites (NZ$10-20 per site) and council freedom-camping sites (free or NZ$5-10). Non-self-contained vehicles need a powered or non-powered holiday-park site (NZ$45-75 per night). Over 28 nights the difference can fund the upgrade to a self-contained 2-berth several times over.
Depot logistics for longer trips
Long-trip drop-off in a different city than pickup (Auckland-Christchurch, Auckland-Queenstown, Christchurch-Queenstown) is standard. One-way fees are usually waived above 21 days on most fleets in shoulder season — ask before assuming it's a flat fee.