I have an ENORMOUS amount of experience with this one (I think I have hauled 6-8 loads of drywall / plywood in mine). I have a 99 ML430. With the seats folded down, a sheet will fit in the truck, but will stick out the back a few inches no matter what. Just how far it sticks out is based on how far up you slide the driver's seat, but even with it all the way up, you cannot shut the gate. Bring a bungee cord and hook it to the tailgate latch on the door and onto that striker on the floor of the cargo area to keep the gate from bouncing all over on the way home. Oh, and another tip: DO NOT PUT THE HOOK OF THE BUNGEE CORD INTO THE DRAINHOLE IN THE MIDDLE OF THE TAILGATE. I know, it looks tempting to do that, but when you try and get the darn hook out, you will put a reverse dimple in the back of the gate which will annoy you till this very day.

Also, you CANNOT lay a 4x8 sheet flat in the back, it will need to be propped up aganst the back tailgate frame, at a level about equal to that of the little cargo net hooks, with the other edge on the floor. Now, how much drywall can she haul? Well, the most I did at a time was 4 "doubles" of 1/2" 4x8 drywall (8 full sheets). I expect that it could hold more in terms of both weight and space, HOWEVER, since you have the sheets propped up on the side against the frame of the gate (on an angle) one on top of another, that bottom sheet of drywall is bearing a LOT of "flying" weight. I expect that 9 sheets or more would be too much weight and the bottom sheet would break. The ML is strong, sheetrock is not.