As we're talking about a long range heavy duty semi, I thought the low center of gravity is not as important, so I imagined the battery packs in a vertical position, behind the cab. The six electric motors would be at least 200hp each and would be in the way of the floor battery pack(s).
To have the battery pack(s) on the floor, between the wheels, just like in the Model S, 3 and X, the distance would have to be bigger between the front wheels and first rear wheels, BUT then the truck would become longer, less maneuverable.
With the battery pack(s) between the wheels it would be the most stable semi truck in curves and Elon would get a bigger bed, maybe a bathroom and fridge too ;)
BUT more importantly, I don't think 6 x 75kWh battery packs will be sufficient for a long range semi, so it's more likely they'll use 12 x 75kW, right up to Elon's seat (and bed). In the Nikola One semi prototype they installed a 320kWh battery pack, which wouldn't be the main energy source, it would just stabilize/complement the Hydrogen fuel cell's current. They rated that truck for a combined range of 800-1200miles, so the 320kWh battery would be maybe 1/5 of that.
As you can see, the hybrid technology makes the truck even longer, as it doesn't just have the motors and battery, but also a Hydrogen tank, fuel cell and a more advanced cooling system, as the fuel cell is only about 70% efficient, meaning that 30% of the energy is wasted as heat.
To have some idea about the battery requirements of such a big vehicle, we can look at an electric Proterra Catalyst E2 bus, which can travel 350 miles with, using its 660kWh battery pack.
So, yeah ... the Tesla Semi Truck will most likely have at least a 1000kWh battery pack, which will give it a ~500 mile range. This would mean that there'll have to be a battery swap station or quick charge station every ~300 miles on "electrified routes".....