Shower/Kitchen Sink Drain clogged

[glistok]

According to the manual diagrams for my 1997 Land Yacht 33, the kitchen sink drain and the shower drain join somewhere above the floor and have a common pipe to the grey tank. If I fill the kitchen sink and drain it, it flows into the show pan. If any water is draining from the shower pan, it is too small to detect. The clog, I think, is right at or after the joining of the two drain pipes.
I have tried a plunger. I have tried a green clog dissolver. I have tried baking soda and vinegar. I am down to the last choice of using a snake. If the diagram is correct, the snake will probably only work by going through a cleanout that is between the two drain systems. The problem is, though the diagram shows the cleanout, it does not tell me where it is. Anyone know where to find it?

Also, if there is a better idea, I am more than willing to look at it.

Thanks!
Geo

[A W Warn]

You might have luck with a shop vacuum. Use the strainers to seal the kitchen sink. Use the shop vacuum at the shower drain. You will need to seal (plastic foam or wet cloth) around the shop vacuum hose at the drain strainer so that it will create a negative pressure inside the shower drain pipe. This might pull the clog backward enough to dislodge it, or maybe completely clear it.

ps:
You will need to seal the air vent(s) for the drain pipe for the sink and shower. (admittance valve or vent - I'm not sure what you have)

[glistok]

Great idea with the vacuum. I have not been able to locate the air vent that is shown on the diagram for the shower. There is also a cleanout shown after the shower and kitchen sink drain pipes join. It has been raining, so I have not been under the RV, but I think that is where I have to go. The plumbing schematic says it is all above the floor. The shower is on the curb side and the grey tank is on the road side. There is no way to get from the shower to the tank above the floor.

[glistok]

OK! I found the cleanout that is last in line before the grey tank. It is behind the storage compartment that is in back of the curb side wheel well. The plug faces down and there is plenty of room for a wrench. It is plastic, and old, so I am worried about breaking it. Just have to go slow.