I use a joe rocket manta bag also. Over the tail I use Kappa Racer saddle bags. They have 28L capacity. While they aren't going to win a beauty contest, they have been very functional. I have toured with them a few times. Then I bungee a backpack overtop of the rear seat. It allows me to have 4 storage places and is enough for myself to go away for a week comfortably.
I was not using a backpack when I took this pic, but the manta and soft saddles are shown. I would recommend getting a tankbag and soft luggage to anyone who wants to go more then just a short ride. I also have an Ogio semi-rigid backpack that is kindof formed nice and slip-stream like. I wear it a ton. I ALWAYS need more room then any bike's trunk has ever offered me.