I drove to Milford Sound with a campervan in January 2017, no problem at all. The drive there was really beautiful.
Seeing snowy mountains and going for a sunny swim in a lake within a couple of hours was quite cool, you'll enjoy the view for sure.
I recommend just stopping on the side of the road and walking around if you're in a nice area. There are some really cool places to be discovered!
Happened to have a lot of bad luck in Christchurch somehow, but the city itself is definitely interesting and offers dope spots for skateboarding and urban exploring. I also enjoyed Kaikoura.
Random story: we picked up a random lonely hitchhiker along the way. He told us he was going from town to town, sleeping under park benches or in public toilets, looking for a dude who had stolen his car all over the south island to fuck him up badly. He suspected his brother-in-law, who had kicked him out of his house before or something. The dude we picked up revealed himself as being a meth dealer and addict who has, among other things, been in jail for beating up his uncle at his dad's funeral. Some really dodgy shit.