[Canonman]

I believe you have both a galley and bathroom grey waste tanks. Our dump valve for the galley tank is just behind the wheels on the driver side. Look for a small "T" handle. You might need a flashlight and need to retract the slide. Probably not marked since the stickers have a habit of coming off. The grey tank dump handle located by the black dump handle is used for the bathroom tank.
Not everyone has had the good luck we've had with the tank sensors. Ours work fine. Looks like yours are working as well.