Roads to avoid? That depends whether you are a competent/confident driver or not. The only sealed road that I don't take my van along is the Elephant Pass (road down from St Marys to Bicheno.) That is because it is particularly narrow, not because of the gradient. You will find that most of the highway type roads have hills with plenty of bends. You will spend lots of time in low gear both up and down hill. You just have to be patient and take your time. Use low gear (auto or manual) on descents to reduce the amount of braking on the way down. What hills have you negotiated in the eastern states?

Fishing. There are plenty of waterways down there where you can use a canoe. Some of the highland lakes have good trout fishing.

We have just spent 8 months in tassy. The only roads we would not take our 3.5tonne 21' van was up jacobs ladder to ben lomond or up mt wellington. Elephants pass is fine if you are competant, as are most of the road up the western tiers, the 99 bends down to Queenstown.

Generators are fine for most freedom camps, just check individually for national parks. 

Plenty of lakes for fishing or angling. Make sure you get a licence. Licence js required for all internal waterways.