It's there, you just got to look for it
Funny this question should come up, because I just made some changes to my Crewmax interior storage and seem to have doubled my capacity.
To answer your question - behind the backseat. I prefer to use soft sided mechanic bags to hold straps and tie-downs and then these stack perfectly behind the backseat. Nobody likes a seat with a backrest that's sitting straight up, so I pulled my backseat forward a bit and reclined it slightly. This gave me some room to work with. You'd be amazed what you can stuff back there. The soft sided bags are key - they conform to what you've stored, you don't waste any space, and they squish perfectly behind the seat.
Here's a pic of what I'm talking about. Note - in this pic the backseat is pushed forward to get a better pic of what I have back there, you can see in the next pic where it actually sits.
All that fits back there with plenty of leg room to spare.
Not too bad, right? Well, I recently came to the conclusion that it's primarily just me in my giant Crewmax cab and decided that I should take advantage of more of the cab's space. By pulling the backseats forward a little more I was able to fit even more back there!
By using some Kelty Camp Cartons/Haulers to utilize this newly created tall and skinny space, I was able to fit so much more back there. Here's what it looks like now:
And I still think there's plenty of legroom for your rear passengers:
I did want to point out that there is one storage cubby in the Crewmax - behind the passenger side of the backseat there's the tire changing tools cubby - they left some room to work with so you can stuff some things in there too. And I'm sure I don't need to point out the obvious - two glove boxes and all the space in the doors, but just saying - try behind your backseat, you'll be amazed what you can stuff back there.