How are you getting from Nelson to Queenstown? Flying? If so, I'd consider flying into Christchurch instead, cutting some days off the North Island, and using them to see the West Coast as well. The route would then become:
Christchurch - Mt. Cook - Queenstown - Franz Josef Glacier - Kaikoura - onward... (Franz Josef to Kaikoura is a long drive, but it's totally worth it; the west coast is gorgeous, and the glaciers really should be seen (with only the scenery around Mt. Cook/Lake Tekapo being more beautiful, imnsho)).
And yeah, Abel Tasman can be done quite well from Nelson.
You could also toss things around even more. Might be impractical depending on how much you've booked already, but with Nelson as starting point (assuming you could pick up a campervan there (if not, you probably can in Picton)), I'd consider driving:
Nelson - Kaikoura (1n) - Franz Josef (2n) - Queenstown (2n) - Mt. Cook/Lake Tekapo (2n) - Christchurch (2n) - drop off campervan, fly to Auckland, pick up another campervan (1n) - Rotorua (2n) - Taupo (1n) - Tongariro National Park (2n) - Auckland (long day back from Tongariro, but doable, especially since you can relax in the hotel afterward) + 2 nights hotel. (On the South Island you could also rearrange things to: Kaikoura - Christchurch - Mt. Cook/Lake Tekapo - Queenstown - Franz Josef - Christchurch.)
That should be the same number of nights as your plan (if I counted right), taking a slightly more measured pace on the South Island (some driving days would be a bit longer, but you're staying longer, too), dropping the less interesting bits of the North Island (Wellington and Napier are "nice", but nothing more; just cities, really - Hamilton is barely that, and I've never even heard of Martinborough), and getting in another gorgeous National Park in return. (Tongariro has awesome hiking through volcanic landscapes; "Mt. Doom" from the Lord of the Rings is located here.) The flight north nearly finances itself by not having to take the campervan onto the ferry (which is pretty expensive, as I recall).