You've picked a great time of year to visit BC.Once Labour day holiday is over the place is yours to visit at your leisure.Why not ask the firm you are renting from to set up accommodation for the long weekend in the Vancouver area and then head out on Tuesday morning when the crowds are gone?That's when all the seasoned(older) RVers get away for a break.The weather is often wonderful til late September and the people you meet are more laid back and easy going.
I think your plan to do sechelt and the sunshine coast will be a great start,then Powell River and across to Campbell River on the ferry.When you come south on the island don't use the inland island highway.Its fast but not as scenic as 19a on the coastline.the coast route has lots of rest stops and neat little oceanside villages to see.Plan to stop in Qualicum Bay at the ice cream parlor with a view of the ocean bay.
At Qualicum Beach go west towards Port Alberni then go right across the island to the west coast to see Long Beach and stay at Green Point (National campground -might need reservations).Drive up to Torino and maybe do a whale watching tour but do plan part of the day to walk the town and get fish n chips at the outdoor eatery.
I could go on but perhaps I ramble.... Charles.