I own a Lowepro Pro Trekker AW 400. It's the largest pack that is still allowed on as carry on luggage. It has more lens compartments than I think you might need, but that leaves lots of room for other things. It also has two huge side pockets for whatever, including a Camelbak, and a removable waistpack/lid.
I've loaded up mine with two bodies, six lenses, two tripods, monopod, steadycam, shotgun mic, gps, food, water, raingear, etc. and gone canyoneering to get to a remote location in Jemez, NM. The pack weighed about 35lbs, but was comfortable and stable all day. I even fell back-first into the river, and everything inside the pack stayed dry and undamaged.
I go hiking with this pack every weekend it seems. I love it. I'll be taking it to New Zealand for three weeks with about $15k worth of camera equipment inside.