As you're considering a sling back, I suppose you don't carry a light backpack for water and snacks. In which case you might consider the
Lowepro - Off Trail 2
I have used it for 10 years. I even hooked water bottles on the belt.
It takes a Minolta X700 film slr + 50mm + 28mm +70-210mm + 35-105mm although I usually did not carry the latter while hiking. I have also used it with a K10D + kit lens + 55-300mm + small tripod (in one of the detachable lens cases).
Disavantage comes when you combine it with a normal backpack as the hipstraps interfere with each other. It can be really uncomfortable with a 55L backpack and this offtrail as the camera bag is partially in front of you (not on your hip).