RDGA is my favourite Alps trip, ridden it twice now - both times in July / August, by the standards of where I live the roads were very quiet, much more so than any decent road in the Cotswolds or South Wales.
Just got back from a trip out to the Dolomites and Italian Alps taking in parts of Switzerland and Austria as well, generally no problems with traffic anywhere.
"Lesser" passes are indeed quieter, but the famous ones are never that bad as to be worth avoiding (IMO)
Alp d'Huez is pretty crap IMO, just a bunch of hairpins covered in lycrists so unless you have an ulterior motive give it a miss, it is likely to be busy all the time, the passes of RDGA will be pretty good from my experience, and the many great passes each side of the main RDGA route will have even less traffic on them. Check out the Journeys book, RDGA is not listed as a route, but look at the chapters "The Grey Alps" and "Some High Stuff" for roads that make up parts of RDGA (and surrounding passes) as well as "The Mont Blanc Region" and "Annecy"
In my 5th edition the trips I would recommend are below (I think they have been the same for a few editions now)
13 - "All Around Mont Blanc" - may be busy in places, but one of the best days out you can have in the Alps
23 - "Around Guilestre" / 24 - "Agnel Sampeyre and Lombard" - I have done most of the passes in these two routes, but never the routes as described, but they should both be great - but a few of the passes are a bit nadgery and the book does not always make it clear how rough / smooth some of these roads are.
25 - "Three Highs" - Or two good roads and a Motocross track - well one half of Champs which is bumpy / tight / falling apart, I really enjoyed it on my Super Tenere, the Mrs was not happy on her TDM, the other side is brand new surface (or was about 2 years ago) so they might have improved the West ramp by now.
26 - "La Bonnette" - Great route as written
The "Gorge du Dulais" is also really nice, not a pass, just a twisty (generally well surfaced) ride through a wonderful canyon.
I would recommend you plot out the routes you like the look of and put together a plan mixing in elements of the trips in the book, this is what I tend to do, if looking at RDGA I would ride from the coast Northwards and stop arounf Guilestre / Barcelonette for a couple of nights to do some of the looping ride outs, and again around Bourg St Maurice so you can do the loop around Mont Blanc (worth it even with a bit of traffic.
IMO North of Cormet de Roseland it gets a bit duller (and busier) the best bit is that middle section mentioned above so if pressed for time skip the most southerly and / or northerly parts.