grey water drain clogged

Just another random thought but if your have wasps they will often get into vent pipes and cause issues. When we bought our used camper I was told that there wasn't a big problem with them in Colorado but in checking the slow drains, after cleaning the "P" traps, I pulled out a wad of nests. Stopped that by purchasing mesh drain screens that are supposed to go in sinks and gluing them into the top of each vent pipe before replacing caps on the roof. Now the only place they can nest is where I can see them and remove any would be invaders. Problem solved.

Whatever fell down the sink drain, is probably not the cause of water backing up in the shower pan. That thing is most likely still stuck in the "P" trap under the sink.

I would suspect a blockage in the grey tank vent line. If that is plugged, the tank can only take so much water, until it gets pressurized. Then the water takes the easiest path available. That path would be your floor drain.

The vent line, in my TC, is a 1/2 inch clear plastic tube, attached to the grey tank, near the top. It then runs up above, where it ties into the main vent. In the past, that line has slipped down below the connection point, and when the tank got full, or close to full, water would get into that line. Once the line was blocked, I would get backup in the floor drain.

Couple ways to clear it. (a) disconnect the line and drain the water. (b) Open the dump valve (into a bucket) and lift the line to drain into the tank.

I used a zip tie to hold the vent line from sliding down and forming a trap. No problem since.

Look for an access panel or door, to view under your floor pan.

While you're taking things apart, might be a good time to clean out the "P" traps under both, the sink and the floor pan. After cleaning, pour a cup or two of water in the drains, to re- fill the traps.

Take the P trap off underneath sink, easy to do, if you move all the TP stored there. HEHE!! And fix the cabinet with some sort of a fence to stop falling items!

If only the sink is plugged, remove the trap under the sink.