Yup, only took 61 posts for someone to nail it.
The gray tank drain is on the side of the tank. When the level reaches the top of the outlet drainage slows. After it reaches mid-point the rate approaches infinity. You can see that the bottom of the outlet is above the bottom of the tank.
The only way you can completely empty the tank is to make sure that the tank bottom is sloping towards the outlet. When our trailer is going to be stored for awhile we put leveling blocks down on the opposite side to tilt and completely drain the tank. We sometimes do that if the dumpsite area appears to slope away from the dump.
Makes for much faster draining.