The warmest places are mostly the far north, bay of islands is spectacular. But also golden bay and abel tasman is amazing and good for swimming.

The thing with RV costs is that you’re coming at the height of the high season. NZ isn’t cheap in general and less so during the peak of summer.

The west coast of the south island isn’t a warm sunny place to live but it is a spectacular place to visit. You should assume that some part of your trip will get rain during summer.