I've taken many trips with a dslr and 2 lenses in a Lowepro Topload Zoom pack. I find the Lowepro brand to be the best. There are various sizes depending on your camera and accessories and some have the AW (all weather) designation that include a cover you can pull over the outside of the pack. This cover is only suitable for light rain.
I attach the camera pack to my shoulder straps just below chest level with two small carabiners and it is always ready to shoot. Depends on the pack and how the shoulder straps are built. This may sound awkward but I have done multiweek trips in rugged terrain in very wet conditions in New Zealand and Patagonia with this setup. Sometimes I place my hand on the pack as I climb or downclimb to steady it. You get used to it. Using trekking poles would be a bit more difficult but then you won't have enough hands to shoot anyway.
I have a Nikon D80 with an 18-70 and 70-210. Nikon and Canon both have 18-200 zoom lenses that could replace the two lenses. If you have an extra lens you can attach a lens pouch to the side of some camera packs, to your hip-belt, in the side pocket of your pack, or stored in your pack until you are ready for more serious shooting.
As for waterproofing I have used a waterproof stuff sack pulled over the outside of the camera pack from the bottom up and cinched around the top. You can also use the Aloksak bags or ziplocs inside the pack or a small waterproof stuff sack inside as well.
I have not found a suitable waterproof camera pack I can utilize attached to my shoulder straps in the manner I prefer. A DIY project I have considered is to glue some closed cell foam to the inside of a small dry bag. Outdoor Research makes a dry bag with a daisy chain up both sides. I could then attack this to my shoulder straps as a chest pouch.
Hope this helps.
Ryan