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.
I'm a photographer and cinematographer, and I'm looking for a backpack that can double as a photo bag for hiking, skiing, mountain climbing. I've found that Lowepro bags are too small. What do you recommend?
A really great starting point is Arc’teryx's Altra 65 ($399). At 4,000 cubic inches, it falls in the sweet spot of pack sizes: it's not too big for an overnight but has enough capacity for a four- or five-day trip, depending on the gear requirements. I also recommend it because Arc’teryx's stuff is extremely tough, and the Altra, which is made from a durable, lightweight packcloth, is no exception. And the suspension system is great.
I've put a lot of miles on Osprey’s Aether 70 ($279). It’s a bit larger than the Altra, and excellent suspension, with a hip belt that can be custom-molded in some retailers. It comes with sleeping bag compartment, room top pocket, and plenty of tie-down points and gear loops. It doesn't have quite the build quality as the Arc’Teryx, but it's a great pack for the money.
Another great option is the Gregory Z65 ($250), a top-loading pack with side zippers that let you access gear that's tucked just about anywhere in the pack without unloading. I’ve found the shoulder straps are just a bit on the thin side, but it still carries good loads. I had no problems with the Z65 on a four-day trip into in the Grand Canyon backcountry with two gallons of water. It’s also designed to let air flow across your back, which is a great feature on a hot day.
I've noticed that modern packs have become over streamlined, and you don’t see many with long zippered side pockets like those on my antique, but still serviceable, Dana Designs Terraplane. They were extremely useful for carrying things like telephoto lenses. And what happened to those removable side pockets that tied into the side straps? They were practical, and they used to be universal, but now nobody makes them. I see a product opportunity.