Take some travel wash or normal washing gel in a container. Look for a Laundromat at some point, also you can rinse items through.
We have done similar trip, Vancouver, Whistler, Rockies - 4 yrs ago - and we took far too much - though it was a lot warmer than expected all the way through - we had expected it to cool off in Rockies but it didn't.
We have a road trip to W USA starting this Friday with our late teens DC and having a similar dilemma, as it can be cool on coast, but 50deg in Death valley!! And I also have a DH and DS who happy to take as little as possible and not wash, whereas DD and I like to be fresher!
You will need a fleece or thick cardigan, but limit to one each, a waterproof but this doesn't have to be a thick walking coat.
Limit toiletries and sun cream etc to starter pack and aim to buy over there, that's what we doing this year. Although if in hotels you will get toiletries anyway.
Where exactly are you going? You will have a fab time, we did, all loved Canada, I would have gone back this year but rest of family wanted USA