It sounds like you had a basin in the sink. If that's the case, and faucet,plus pump switch was on the basin would fill up and dump much of the water over the edge of sink. More than what would go down the drain. The weight
of the water in the basin would more than likely prevent water from going down the drain also. I keep a plastic basin in my sink also. Helps me monitor what goes into the gray tank.
Hook up the city water connection and check for leaks
in the sink area. If you don't find any then partially fill fresh water tank, turn on pump and check for leaks
. There is a bypass for the water heater on mine, but not sure on your year trailer. That wouldn't have any effect on water in the sink area. Please let us know what you find. Remember when hooked up to city water you don't need to use the pump. The pump is for the fresh water tank when you don't have city water hook ups. I've caught the wife turning on the pump when we were hooked up to city water a couple of times. Most of our camping is without hookups so it was force of habit. Do you have a lighted pump switch?