Almost all of the drain lines and pressure lines in an Airstream run on top of the floor. There are some instances where they run pressure lines under the floor. They are few and far between. You mention that the only time it leaks is when you use the sink. Have you tested the drain with a bucket of water (not running a faucet)?
In many of the trailers there are drain valves that drain thru the belly pan. Usally there is a valve that allows you to open up the lines and drian out all of the water. These valves can be on top of the floor, or in the belly pan itself. They are usally capped with a red handle. The reason for my description is this. IF your trailer had the belly skin replaced and the drain line was not routed thru AND IF you have a leaky valve or it was not closed complety then you would get water out of the belly pan with no visible source of leakage.
Another way to see if this is the case is to turn on the pump and leave it on. Sit on the couch or bed with a book and listen to see if the pump has to keep burping or running in short bursts to keep the system pressurized.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato