Grey Water & Black water tank capacity??

just wanna make that clear i know my 2720 sl it has a standard  30 gallon grey water but is  that include the black water tank also ,the reason is i want buy one of those Tote Tanks and want to make sure is the rite size ,Thx for your input,also if you know any place to buy those that has a better price them camping world i appreciated,using a lot state park and i may need one ,thx again. ;D