First of all do not be mislead by the comments about the rough roads, that only applies once you leave the main roads, which are fine.
The lake you are referring to north of Campbell River could be Morton Lake, which is a nice spot, good fishing and nice campground.
My favourite campground on the Island is Buttle Lake in Strathconer Park, which is on the way to Gold River from Campbell River.
Further north, Cluxewe campground near Port McNeal is nice, on the ocean and run by the local Indian Band, and in Port Hardy I always stay at Quatze River which is about a mile from the town.
On the road from Port Hardy to Port Alice is a forestry campround at Marble River, rustic but good fishing.
In all of this area watch for bears and take appropriate precautions.
Hope this helps, if you need any more info get in touch.
2008 Ford F350 crew cab short box PSD, 2021 TravelAir 90W camper