Hello Saraian,
We travelled around for three weeks in a campervan during August in NZ years ago. It was cold, especially on the South Island, but not too cold. We only stayed in a camp ground in Queenstown for two nights. Okay, some mornings the van was frozen, but it is well equipped and we did not have a problem.
You just start the van drive off and it is getting warm!
I would do it again.
Regards,
Anita