How to stop drain pipe from leaking on a 2014 shower 1985 lancr travel trailer

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 -

https://www.ebay.com/itm/SHASTA-VINTAGE-TRAVEL-TRAILER-OWNERS-MANUAL-NEW-COPY-/261306882236

You have a blockage somewhere.
First place I'd try is the tip of the faucet.
They screw on, and can get clogged with sediment.
God bless your efforts.

The first thing to do is to drain the fresh water tank. Drain the hot water heater. Install the RV antifreeze by allowing the water pump to be pumped into the water lines by opening the valve for this process. Open each water valve on all the sinks and the toilet valve as the RV antifreeze is being sucked into the water system and run until the color changes to the color of the antifreeze. Allow each trap to have antifreeze in it also, sinks, shower drain etc. Remove the inline water filter if one is installed and install the bypass plug in its place.

I actually am not able to get rid of the antifreeze taste from the water system so I blow air into the hot water tank drain at a low pressure until all the valve at each place water is controlled. Do not forget the outside sprayer and the shower hose and the shower head.

I allow the pressure to build up to 20 psi or so and then go into the unit and open each faucet until all the water is pushed out of the lines. Then I dump antifreeze into each P trap. Do not use the antifreeze used in the engine cooling system but the non toxic type made especially for RV water systems.

Look under trailer for red and white capped off lines. My motor home drains are the outside shower valves

check the piping to the shower for damage, kinks , blockage
check that the washer in the tap is not expanded and blocking off the flow of water

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.

In some cases there will be a small panel on the front of the shower pan with a screw on each side so that you can gain access. I have seen a few that had access from the back side (in a closet/pantry) and some may be accessible from the under storage. If not, then the entire shower has to be removed (I have actually only had one that I had to remove the entire shower to repair the drain). If the front of the pan has what appears to be an access panel but it has not been cut out then that is an option and you will have to fabricate a new cover. Just be extremely cautious when cutting the front to make sure that you do not cut into the pan. Once you gain access to the drain there will be a tail piece coming down from the shower pan and will slip into either and "S" or a "P" trap. I have seen where the tail piece was cut too short and as a result became disconnected and caused the leak, I have seen the jamb nut not tightened on the trap and I have seen the trap actually cracked causing the problem.

You may have a very small access panel on the front of the shower pan or through an access point in a cabinet on the backside of the shower wall. I have seen the shower tail piece get cut too short and as a result is becomes disconnected from the "P" trap under the shower.

The best way to repair is to completely remove the shower surround and pan. Build a better support and repair the drain then reinstall. Leaking water will ultimately ruin the actual floor of the camper.