Were is the drain plug on my holding tank on my 2003 mobile scout 5th wheel.

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

You’ll have to remove the coreplas protective barrier from the underside.

There should be a compartment on the outside, typically where most utilities are connected, that will have the drain connectors for black and grey water. Black is for your toilet, grey is all other water water. Look for the labels to ensure you have the correct one. Depending on the manufacturer, it will be either a black, plastic or silver, metal T-handle. Once located, you will pull the T-handle out, approximately 4-6 inches (10-15 cm).

The RV sewer system should end in a 3", 31/2" or 4" black plastic fitting that has 2 to 4 plastic tabs that a flexible hose will lock on with a twist. The fitting probably has a cap twisted on. After the sewer drain hose is connected to the RV drain, and secured in the sewer, a 'T' handled pull rod should open the gate valve. Most drain the black tank first, then use the grey water tank to flush the hose clean. If the 'T' handles aren't in your sewer compartment, you'll have to look under the bottom edge of the RV to find them.

You'll probably have to crawl under and trace where they go. You could possibly put some dye in one of the grey water tanks (ie: the kitchen sink drain or bathroom sink or shower drain) to figure out where it comes from. The black tank is your toilet drain. It always has the larger diameter pipe going to the valve (usually 2 or 3 inch diameter) The grey water tubing is usually 1 1/2 inch tubing.

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 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.

On some later model Montana 5th wheels, there are actually three holding tanks which will eventually require draining. (two grey tanks, 1 for the kitchen sink(s) and 1 for the bath tub & bathroom sink, and one black tank, obviously the toilet...) The two most used drain valves (1 black & 1 grey) are together in the docking station area where the city water supply is hooked up, the other (kitchen sink) is at the left rear of the coach behind the farthest rear wheel along the frame. (That is where "Keystone" installed the grey tank for the galley, or kitchen if you prefer.)
If certain it is the black tank which won't dump when the black tank valve is pulled, there is a restriction before the valve (or the valve itself is not opening hence a broken cable to the valve?) One might try "snaking" up into the sewer line, measuring the distance from the outlet to the valve location, assuring it is either open or closed. Once that has been determined, you will at that point have to remove the sewer line at the valve for repair. Be sure to wear latex gloves and eye protection and have a drain pan or bucket handy for spillage which can be collected and dumped down your home toilet or outside sewer clean out if you know its location.

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.