Is yours a diesel? On mine there is a valve in the water compartment that drains your tank and water lines. If your goal is to get it totally empty, suggest opening the valve, use your pump and open sink faucets. If you have levels, raise the right-side so its tilted and let it sit that way for several hours.
The drain valve is in the front left side of the water compartment and has a tube exposed on bottom. This is the low point drain and also an entry point for bugs so make sure its sealed if not already.