On a Scamp
the toilet is sitting directly on top of its balck tank inside the bathroom itself.
Then the black tank output goes down and into a dump valve so it can drain into a sewer either with the grey or not depending on how many dump valves there are there?
The grey from the shower goes into the pump and then onto the grey tank under the trailer.The grey from the kitchen also goes into the tank and the tank output then goes into the dump valve manifold again unless they are 2 separate lines.
I have seen it both ways.
In order for the grey to dump onto the ground something is not connected somewhere or the dump valve is open.
The pic you show only shows the pump input but I can not see where the output goes if it goes anywhere at all?
Also is the bathroom in the front of the Scamp
or on the side??????