British Columbia

Coming this far, you should try to visit the Icefields Highway between Jasper and Banff/Lake Louise area.
In Victoria, Vancouver Island, visit Butchard Gardens, beautiful gardens and should be bloom in your time frame.
Capilano Suspension Bridge is a good sight along with short walk through the large timber.
Hell's Gate is a sight. It will not have large amounts of water during your time frame. It is a wonder during high run offs.
Stanley Park in Vancouver.

Google these areas and have a look, the West Kootenays is about an 8 to 10 hr drive from Vancouver ( Lower Mainland) in an RV without doing much but driving and refueling. This is not to say you wouldn’t take a few days camping on your way in and out of the Kootenays. So google these places:
Nelson BC, Kootenay Lake, Kootenay Lake Ferry, Kaslo. Paddle wheeler Moyie, Ainsworth Hot Springs, Halsyon Hot Springs, Nukusp Hot Springs, Idaho Peak BC, New Denver BC, New Denver Centennial Park Campground, Silverton BC, McDonald Creek provincial Campground, Summit Lake Provincial Campground, Three Islands Resort ( campground) Syringa Creek Provincial Campground Sandon BC. Nukusp BC, Valhalla Provincial Park, Slocan River , Cody Caves.
All the campgrounds that I listed are on Lakes
Soup

I would highly recommend the Okanagan Valley (south-central BC) as part of your tour, especially if you are a wine lover. It is an area of long, narrow lakes in a glaciated valley, with hillsides terraced with vineyards, beautiful flowers and fruit trees everywhere, and wineries with great food as well as good wine--makes me want to be back there just thinking about it. And you will be there at the perfect time to stop at roadside stands selling fruit.

There are some nice provincial park campgrounds in the Kelowna area, although they are often very full during summer. Early autumn might be less crowded.

I second the okanagan valley. Beautiful. We stayed at wrights beach cg which was right on a lake in Pentitcon and also stayed on a lake in Vernon. Visited wells gray provincial park in Clearwater which was gorgeous.
Have fun !

Three weeks is ample time to do a circle tour that would include the Alaska Highway in British Columbia's north. This would include a quick dip into Yukon too, near Watson Lake.

The Cassiar Highway on the northwestern side of B.C. is one of the best routes for seeing bears and glaciers.

The Alaska Highway between Watson Lake and Fort Nelson is where the caribou, bison and bears hang out in late August or early September. As well, Liard Hot Springs provincial park is a great spot to spend a couple of nights soaking in the pool and relaxing.

The Cassiar and Alaska Highways have terrific provincial campgrounds. The Cassiar has three by lakes (Meziadin, Kinaskan and Boya) and the Alaska Highway has Liard Hot Springs and historical Kiskatinaw, as well as two campgrounds lakeside at Muncho Lake and another lovely one at Summit Lake. Watson Lake also has a gov't campground that is very nice BUT we usually opt for the commercial campground here to clean up and stock up at the nearby grocery store.

The Okanagan can easily be included northbound or southbound, via Hwy 97.

Having said all that, I must admit to also greatly enjoying the Kootenays in southeastern B.C. Very scenic. Relaxed. Good campgrounds. Soothing hot springs.

Thanks guys:)

Our son has brought over some mapbooks of SW area, but looks as if I need to get some for further East (& possibly North). It is a huge area (compared with Scotland!)and although we want to see as much as we can in the time, we also want time to relax and enjoy the areas we visit. We made the mistake the first time we went to Europe of travelling too much and passing places by. (6000 miles in 6 weeks) We are putting a limit of about 2,500km on the trip which we think should give us enough time for both travelling and relaxing.

I now have lots of places and campgrounds to look up and will continue my research.

BTW we arrive just before Labor Day weekend - is it necessary to book ferries and campgrounds before we arrive?

The problem with labour day weekend is that a lot of the campgrounds will be full or nearly full as most Camper People in Canada go out for the last long weekend of the camping season. The other issue with labour day weekend is that a lot of the provincial and National park campgrounds close for the winter just after the weekend. However these campgrounds are mostly in the interior on BC and the mountainous areas. Vancouver area and Vancouver island as well as the Okanogan Valley are the some of the areas that will see the campgrounds stay open longer.
I want to stress that not all campgrounds will close, most privately owned campgrounds are still open, it's mostly the ones in the provincial and national parks.
I would recommend making reservations for labour day weekend and do it now. After that weekend reservations usually are not required however it really depends where you are staying and if you want to ensure that beautiful lake side site etc.
Soup.

Also best to avoid the ferry on the long weekend, especially the Friday afternoon, Saturday morning and the holiday Monday. Lineups and waits for the ferry - can take 3-5 hours to get on a boat on a long weekend.

You can make a ferry reservation if you wish to go during those periods, but as Soup mentioned, the Island will also be busy with locals enjoying the long weekend. Provincial park campgrounds will be full too.

Thanks SideHillSoup and sue.t

We were thinking that if the weather is good we would start with the Sunshine Coast (does it live up to its name for Labour Day? - the only time we were in that area before was October and it rained like it rains in the west of Scotland:( The waterfalls were good though:B) and cross from Powell River on the Monday or Tuesday. If it is not good weather we would probably start by going East - so booking too far in advance is not really an option.

We are not averse to a spot of boondocking - in fact over here we spend all our time "wildcamping" - but we understand that it is only possible on public land. Could you confirm what this actually means? and are there any spots that you would recommend. Our rental contract indicates that we have to stay on paved public roads, so I assume logging roads are off limits.

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.