2016 GMC 3500HD 6.0L gas undercarriage "box"

[eurobound]

I'm sure this has been asked before but I can't find anything on google other than a DEF for diesel engines. Trying to figure out what is this plastic "box" or large cover under the truck on the gas trucks? The one under the driver side that runs pretty far to the back of the truck opposite side of the exhaust?

 

I'm stumped!

 

It's a 6.0L gas MY2016 3500HD crew cab truck.

 

May sound stupid but I'm looking to cut a hole above it, under the rear seats, for an Infinite Baffle subwoofer setup and need to know if there is any room above it and what I'm dealing with below the floor.

 

Any thoughts and comments much appreciated!

[eurobound]

Nevermind... I assume that's the gas tank. For some reason I was expecting to see something on both side of the truck to equal out the weight of the gas and that's why I initially dismissed the fact this was just the gas tank. 

[CamGTP]

Yup, you are correct, that is the fuel tank.