Grey water in Shower Pan

Thanks Everyone for your lightening fast responses to my question. Bumpy, I'm pretty sure there is only one grey tank because there is just a very small kitchen sink and bathroom sink and shower to drain into it and they are all in a line on the same wall of the trailer. There is only one grey tank on the fill monitor. I'm pretty sure the shower drains into the grey tank. We've done stuff like add soap to the water in the sinks to make sure that what we are seeing in the shower pan is the actually coming from the sinks. Tenbear, the trailer is new to us. We just bought it privately - 'just' is the operative word here. Your thoughts about a possible drain blockage after the sinks and shower join and before it enters the tank are similar to mine. I can't figure out how to get at it to check though. A snake doesn't seem to be helping but maybe I'll give it another shot - maybe with a new and better snake. Your suggestion about back filling the tank is probably worth a try. We got it about half filled when we were back flushing - mostly because we were trying to test whether the monitor was even working -0 which it was. Then we emptied it. I'll try again tonight and fill'er right to the top to see what happens. Camp Forrest..I get what you mean about the vent and that was our first suspicion also but running the sinks with the drain open or closed makes no difference. Too bad , eh? I'll post later with the 'results' of tonight's trailer fixing event. Thanks again to all of you!
Sue