First - welcome to the forum! Galley = kitchen. One grey tank light is for the galley sink water and the other for the bathroom shower and sink water. Black tank is for toilet. If you have 2 bathrooms you might have 2 black tanks hence the switch. You might have to search for the other gray tank handle, ours is under the rear slide which I probably would've never seen if it wasn't pointed out by the technician during our PDI.
2020 Flagstaff Micro Lite 22FBS