How can I properly support a raised concrete patio?
FWIW, that's technically called a "raised patio" rather than a deck. At 40' long, your challenge will be to prevent cracking...so I'd count on putting in expansion joints. 

If you want to ensure a solid base, you need to use crushed rock and add it layer-by-layer mechanically compacting each layer before adding the next. That's going to be some work to get up to 12", but certainly doable. You'll likely raise it in 3-4 layers. 

At that point, you're essentially building a slab-on-grade foundation. So you shouldn't need any actual foundation walls. 

All that said, check with local code first!

Note, however, that if you're ultimately capping with stone, you really don't need all that concrete on top. You need a retaining wall to retain the layers of compacted rock, but you can then place the stone right on top of that on top of a layer of sand.