Surely you dont need to carry your 55-250 and your 135-400 at the same time?
I dont really use backpacks, i just throw a lens or two into my pockets depending on what I might need, but when I do need more than my pockets can handle, I use a Berghaus 30 or so litre backpack. Its not designed for cameras, but it is designed for hiking so it does have little knick-knacky pockets for filters, cards etc. It is also incredibly well waterproofed and comfortable to wear.
I couldnt find my exact rucksack listed, but I guess similar is the http://www.berghaus.com/ProductDetails.aspx?ProductID=2537&Gear=2