My light switch for that light is on the dash. My basement compartment light switch is next to the door.
The drain for the water tank will be at the lowest point the water system is at. Usually a pipe will be lowest. Your tank will be higher. Look under the rig near the area where the tank is. You'll probably find a plastic hose (1/2" diameter) sticking down through the compartment floor. Follow that up into a compartment and look for a small plastic valve. Sometimes these are turn type, other times you pull them up to start the drain flow.
But it seems like every rig is different...so...you'll have to just look carefully and try to think like an overworked, underpaid, engineer.