Trailer diagnostic panel

is your fresh water tank full it could be a overflow or a low point drain

If you had both tank dump valves opened and the same time, raising the hose above the top of the tanks could allow liquids to mix. Gray water can smell as bad as the black tank, so you can't depend on odor. Best practice is to dump the black tank first, close the valve, then use the gray tank to flush the hose out.

the grey water is your shower and sink drain tank. the black water is your... you know. the two combine to the dump that's held together with a bypass for grey water to flow into the black tank and vice versa so you can flush the black tank further. all of this comes out the end into your dump out for black and grey. If your talking about white water (potable) holding tank, filling this drains through the use of it, run the shower outside or hold flush down until it drains. otherwise. the hotwater tank will drain with the anode when you regularly change that once a year as directed by people who camp once a year - change that anode -

The galley holding tank holds the grey water from the galley.

U SHOULD HAVE A BLACK TANK AND A GRAY. TOWARDS THE BACK WERE U HOOK UP THE HOSE FOR DUMPING U WILL SEE TWO LEVERS ONE IS FOR BLACK ONE IS FOR GRAY. AND IF UR TALKING ABOUT FRESH HOLDING TANK LOOK UNDER NEATH AND U SHOULD SEE A PLASTIC NOSEL WITH A TURN VALVE LIKE WHAT BOX WINE HAS TO PURE A GLASS

If you're talking about the drain valves, most RVs have them on the left side of the RV in the same compartment with the water and electric hookups. There should be 2 lines, one that is almost twice the diameter of the other. The larger line is for black water and the smaller one is gray water. You should always drain the black water tank first so that when you drain the gray water it flushes the really nasty stuff from your drain hose.

If it is a big valve it is your sewer tank drain valve. There is two valves. One is the Black water drain and the other is the gray water drain. the black water drain comes from the toilet holding tank, and the gray water valve drains the shower and sink holding tank.

You have the question we are all asking.
rv tank gauges never seem to be very accurate. The best you can do for your black water tank is to try to see where the sensors are mounted and try to spray water on them to loosen any gunk.
I have had 4 motor homes and never had very accurate tank indicators.
Your black tank can be checked by taking a flashlight with the water off and looking down the hole of the toilet. When it gets close to the lowest part,of the drain hole it is time to dump.

Fresh water holding tank will have a single 1/2" line hanging down under the trailer. It will have either a cap or a valve. To drain open the valve or remove the cap. For gray water and black water you must be connected to an approved dump site with an approved RV sewer line. Open the 3" valve first and allow the black tank to drain then open the 1-1/2" valve to drain the gray water. This will flush the sewer line as well.

level sender stuck