The gray and black tanks start in the front compartment, pax side, and extend the full width of the MH. If you remove the plastic liners/shrouding you can easily see the tanks. They are pretty shallow. The black flush fitting is on the pax side and the sensors are on the driver side.
In the pix below the gray tank is the aft one, black forward. If you look closely you’ll see the flush fitting where it enters the tank.