Try the Billingham Hadley/Hadley Pro (Pro has a handle as well as a strap). It's pricey but it has a padded insert that pulls out completely to leave you with a soft canvas bag with 2 front pockets, plus a zipped rear document pocket. Get one with the optional shoulder pad and it's a great little bag. The big plus is you don't have to unload all the kit, just pull out the insert when you want it as a normal bag.
My Hadley Pro carries 40D with 17-40L, Sigma 18-200, Canon 70-300IS, Nifty-Fifty Mk1 and Kenko 2x converter quite comfirtably, with a few accessroies and spare batteries in the front pockets. You can also get add on pouches that clip onto a leather band which surrounds the strap at each end. I got 2, one carries a 350D body and the other a 580EX. All that plus it's still airline cabin-safe size with the pouches on.