I am General manager of a leading online Motorhome booking agency based in New Zealand called Motorhome Republic, so know a bit about excesses and bonds and CDW options on Motorhome and Camper hire in New Zealand as we deal with all of the companies.
As you have found all of the Motorhome companies like to play with excess and bond levels and set them very high in New Zealand and Australia in most cases. They all self insure like the car rental companies where if you take their own CDW options for $20 - $40 per day you can get the excess and bond down to half or $0.00 liability.
We may be able to help you with independent cover though as we have sourced and created Drive Easy Covers which can cover excesses but typically only up to around NZ$3500. So this may not be suitable if you are renting a vehicle that has an excess above that. We are working with our underwriters to create some more encompassing policies. Our policies are designed to give customers a lower cost option that those offered by the rental companies themselves.
Also note where you are using a cover sourced outside of the rental companies own reduction policies the rental company will still expect to take their full security bond amount which can be up to $5000 - $7500. This is taken as a pre-authorisation on your credit card during the duration of your hire and the money released back to you on safe return of the vehicle undamaged. So if you are using an independent cover make sure you have your credit limits on your card extended so as to not need this money during your holiday.
One of our team would be happy to talk to you further.