If you're talking about the Chevy Avalanche, you can build a small box to fit under the rear seat, You can still do the change feature as well but the seat will not quite go all the way down. I have a 10" under the rear seat of my 2003 Avalanche, I do not think a 12" would fit, a 10" is already really tight. The box I made is 11½" x 11½" x 6" I think anyway. There are very few speakers that can run that small a box it's something like .36 cubic feet. I found a 10" Phoenix Gold Tantrum to be a decent fit as it claims it can go from .20 - .55 cubic feet on sounddomain.com.
If you're talking about the boat Centurion Avalanche, try to place the sub under the drivers seat. That is a very common place for a sub in a tournament boat.
This is the old speaker I had (MTX Thunder 3000) new Phoenix Gold Tantrum sounds much better in this box.
Picture with seat in normal position