I'm pretty sure you have 2 gray tanks. Look more closely for them, and for me, forget the sensors - find the handles to the tanks, drain them and then log how long, how much you can use it in a normal fashion before draining.
Mine has the black and bathroom drain handles close together and labeled on the side. My galley tank is further back down the side (with a little label saying there's one there) and it sticks out thru the side of the frame. I figure you will find it hidden somewhere up around the frame and probably in front/behind/in back of the tires.
Get a clear connection/elbow for your sewer line once you find the valve, wait a day or so (if everything is showing full) and dump the black tank first noting how much came out (you DO want it to be pretty much full so you don't leave solids just sitting). Run your black tank flush for 15 min. or so then dump each gray tank and note the amount of water coming from them. You'll probably have to get under the trailer with a light to find the handle.....or it's fell off.
