If you have two separate holding tanks, gray and black, then the gray water drain line from the tank to the fitting for the dump connection is 1.5" ID plastic pipe. The black water drain line is 3" plastic pipe, same as in most homes. If they are black, they are likely ABS. If they are white, they should be PVC.
Anyway, trace them both from the dump fitting back to their respective tanks. Somewhere along the way there should be a slide valve for each. On some of the newer trailers, the dump valves are way under the TT and are operated by a long rod.
However on older units, the dump valves were usually fairly close to the final dump fitting where stinky slinky connects. On my '99 2453, the gray valve is just to the left and the black valve is just to the right of the dump fitting:
If the system has been modified, it is possible that they plumbed everything to a single valve. That is not good as there are times where you might want to leave the gray side open such as when you're in a full connection campsite for an extended period. Also, it is a very good idea to be able to dump just the black first and then the gray which acts as a bit of a rinse for your drain hose and ensures that all solids are moved out of the hose before you disconnect and rinse it.
It is also possible that the gray drain line is completely clogged. Fixing that is not difficult. But don't use draino or any other unclogging chemicals.
What would be best is if you could take some pics of what you have and post them here so that we can see exactly what you are describing.
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