I agree that Michelin maps didn't show all sites but in my experience they showed enough and negated the need to do research beforehand and / or carry heavy guide books.
However, the Michelin maps I have just taken delivery of are much nicer than the old ones and show much more detail thanks to the better scale. Although they don't show camp sites they do show voies verts (green ways) which we are particularly interested in.
One solution would be to set out our route on the maps and then mark them up with the camp sites mentioned on the web and in the guides. Could be a nice winter project.