It would be a hassle for a Canadian to buy and insure a US used vehicle in Washington State, let alone someone from overseas who's a friend of the Canadian. Also, check carefully on the coverage on Canadian insurance for driving in the US.
I hope everything works out. It will be a great adventure. My only caution is not to overdo the daily drive distance or underestimate how far it is between places. Many folks limit themselves to 300 miles in any day and don't drive that far every day - you don't get much sightseeing except from the RV window!