For my trip to India and Nepal in 2012 I used a large North Face Base Camp duffel combined with a Lowepro Photo Sport 200AW as my day pack. It is a versatile combination and was possible to carry both when fully loaded. I used Sea to Summit eVent compression dry sacks for storing clothes + REI down jacket + Marmot Lithium 0 degree down sleeping bag. The Photo Sport is comfortable and able to carry a full frame camera such as the 5D Mark III with 24-70 f/2.8 and 70-200 f/2.8 (though it's quite cramped in the camera storage area when using f/2.8 lenses... I opted to bring f/4 lenses instead), hiking poles, snacks, first aid kit, 1L Nalgene bottle, batteries, etc. I also went from central India to the Himalayas and I was able to bring enough clothes with me in the duffel to readily handle the different climates. The real challenge for me was keeping luggage weight down for flights. Airlines over there have more restrictive weight restrictions and they will sometimes weigh your carry-on/daypack. I weighed everything with a digital kitchen scale before going and made sure that my gear wouldn't exceed the weight limits for each airline I would be flying. If you want a copy of my planning spreadsheet or any other info, send me a PM or email. Best of luck with your trip!
A couple other thoughts... I didn't have spare room in the duffel to stow the camera bag, so whenever I was traveling to a new location I carried the duffel on one shoulder and the camera bag on the other shoulder. That worked but was not ideal. That said, I was able to comfortably hike over 100 miles with the daypack itself. Also, if you are traveling via bus in the middle of the night with your gear on your back, wear a headlamp when you disembark so you can see the ground... I misstepped on broken asphalt and badly twisted my ankle when I got off a bus at the start of my trip in India. Having all of that extra weight on my back did not help. Learn from my mistakes!