Don't get a larger pack until you really try to make this one work. We use a 55 liter pack for trips up to eight days...

One option would be to tie either the bag or the tent horizontally underneath the backpack...or if you prefer, on top of it.

But once you get your tent and bag in there...how much other stuff do you need to bring for an overnight trip? a little food and a stove, and you are good to go!
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Check our our website: http://www.backpackthesierra.com/

Or just read a good mystery novel set in the Sierra; https://www.amazon.com/Danger-Falling-Rocks-Paul-Wagner/dp/0984884963