You’re right, Mark: even though Lowepro and Tenba make some great camera backpacks, it's hard to find a dedicated camera bag that can really handle a multi-day load. You'll want to purchase a true backpacking or climbing pack and then adapt it to manage your photo stuff.
Look for a pack that's at least 3,800 cubic inches, and maybe a bit larger. You'll have enough room for a compact camping or climbing load, and enough space for several pieces of camera hardware. You’re probably packing a DSLR, a large zoom, a wide angle, and a pro-level digital camcorder of some sort. And chances are your bag needs to be tough: with all that glass, you'll be hauling a heavy load.