Sounds like a fine trip. Maybe a few questions--this isn't my most travelled neck of the woods but I've covered most of that territory at least once or twice.
How are you getting from Vancouver to Williams Lake? There are two routes (mainly), one via Hwy 1 (Trans-Canada) through the Fraser Valley and the Fraser Canyon, then up 97 from Cache Creek. I've grown to loathe the Fraser Valley (horrible suburban sprawl and a stench of chemical fertiliser) although the Canyon remains interesting. Other route is Hwy 99, comes out on 97 just north of Cache Creek. Goes through Whistler (also loathe--way too crowded) and Lillooet. I think it's great country and lots of place for either dispersed or Forest Service camping. Road from Vancouver to Whistler is really annoying--rich endorphin junkies driving like maniacs on a windy road. Whistler east, it looked as though people were catching fish at the FS campgrounds. Nothing spectacular, but fishy.
Camping in Vancouver is a nonstarter. If you REALLY MUST camp there, I'd reluctantly recommend Capilano Campground, run by the First Nations Folks there, just under the Lion's Gate Bridge. It's crowded and expensive but clean. Provincial Parks in the area fill up fast, and are helliferous expensive. Just forget Porteau Cove, it books up months and months in advance and there's a really noisy train goes by at night, anyhow. Near Whistler Cal-Cheak Forest Service Campground is pretty good. I've only camped there in the fall and have no idea how crowded it may be in June.
Lots of nice dispersed and FS camping between Cache Creek and Williams Lake. I'd gather you'll be travelling Hwy 20 from WL? An adventure. Most of it is really beautiful, some nice camping and some good fishing. Got clobbered by pine beetles and forest fires in recent years. The famous part it is the stretch from Anahim Lake to Bella Coola which has a 10k stretch of 18% (yep, really) grade on a narrow, unpaved road. It's gloriously beautiful and there are porta-toilets at the top of the hill. I recommend using them, you may spare your upholstery. The other thing that one should know about the western extreme of 20 is that there are LOTS and LOTS of grizzlies. More than I've ever seen anywhere else. Saw 8 along the road in the space of an hour and if you see 8, reckon on there being lots more keeping a low profile. This may have implications for how and where you camp.
Are you then taking 16 back to Prince George from Prince Rupert? I agree with Frank about the campground although I may have read recently that it's being closed down for "development." Snarl. I've eaten the freshest and best halibut ever from the fish store in Prince Rupert. Just grilled it back at the campground. To die for. Hwy 16 is pretty magnificent. North of Terrace, I camped for a couple nights once in May in Redsand Lake FS Campground. It's some years back but it was quite nice and I think it's supposed to be fishy.
Hazelton area and the Ksan village are interesting. Some great carving and buildings made by the Gitksan and Wetsuweten folks. Otherwise kinda depressing due to a lot of poverty. The Hazelton Elementary school is the only school I ever encountered where the kids had ASKED for an inside recess because there was a griz wandering around the schoolyard. I'm sure it happened elsewhere too, but that was the one I met up with.
Frankly, I'd likely give Prince George a miss. (You can't, actually, but I wouldn't hang around.) Probably a bias because I got food poisoning at the Pizza Hut there...But there are some nice fishy lakes around Quesnel including Quesnel Lake itself and some ok camping. I also camped a few nights at Green Lake, east of 70 Mile House, reputedly fishy. But becoming rather developed, I gather.
More may well spring to mind. I sound rather negative about some stuff above, but I love BC oh so deeply and envy you your trip. For eight years in the early 2000s I had a job that required me to visit schools all over the Province for a stretch in April and May. Spent about 6 weeks on the road, each year, camping in a VW Westy. Got to know the territory somewhat.
Enjoy. I'll try to answer questions or find someone who might know if I can help. I'll look forward to the trip report.
