I'd suggest north on I5 to the border, then jump over to Hwy 1 via. BC 15. Follow 1 west then on to Hwy 99 north to Whistler. You didn't say what size rig you had, but road is good thru to Whistler. When you leave Whistler, 99 goes north to eventually connect with Hwy 1, but I would not recommend it - narrow / twisty, & rough !
I'd head back south, connect to Hwy 1 eastbound, and follow it to Hope, B.C. ( Try to time your travels around Vancouver area between 9 and 3, as rush hour is crazy. Once in Hope, you have 3 choices - Hwy 1 north, Hwy 5 north east, or Hwy 3 east. Hwy 5 is the fastest although it has some long grades. Most of the semis take 5. All routes wil get you thru to Alberta. Hwy 5 connects back with Hwy 1 near Sicamous, B.C., then follow Hwy. 1 east to Banff area. Once into Alberta, you have a choice of highways to get back down to the US.
As for things to do in Whistler, they do have an RV park there. There is an amazing Gondola ride that runs peak to peak. Mining Museum in Britannia beach. Lots of restaurants and tourist stuff in Whistler.
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