The Inside Passage is a stunner, part of the same route that the cruise ships take to/from Alaska.
16 (the Yellowhead) is scenic, period. Have a look at Seven Sisters (
http://www.env.gov.bc.ca/bcparks/exp...pgs/seven_sis/). Also...
http://tranbc.ca/2012/09/26/the-tran...ed-highway-16/
Off 97...Barkerville, which I mentioned earlier, and Bowron Lakes and Wells Gray:
http://www.env.gov.bc.ca/bcparks/exp...pgs/bowron_lk/
http://wellsgray.ca/
You'll always be able to find spots in privately owned campgrounds, but generally they are not as scenic as the provincial parks.
The Provincial Parks reservation system has received much attention here over the past couple of years for being overly restrictive. The locals feel that they're getting shut out by tour companies like those motorhome rental outfits working the system to book up blocks. They've tried to make it "fairer" by imposing limits--i.e. you can only book a few months in advance (I think it's a 3-month window), so if you want a popular destination you do need to plan ahead. For some of the back country hiking they even limit the number of people allowed to hike in and camp for environmental reasons.
April/May might not be as busy but it's bound to be colder and wetter, or even still covered in snow if you up high (elevation and latitude). You probably won't get summery weather until later in June in the more northerly locales.
BC is BIG so 6 weeks is a great time frame. Prince George is only halfway up the province. In size, the area of BC is equivalent to about the same area as California, Nevada, a third of Oregon, and a bit of Idaho all put together. There's a cool site called Overlap Maps--plug in Canada-BC over US-California (
http://overlapmaps.com/).
Sorry, sidetracked there. My point is, don't underestimate driving times.