Water leak into grey tank

[Ongeman]

Upon the first use of our new trailer I noticed the galley grey water drain flowing water out continuously whenever water pump was on or city water was supplied. No faucets are on! Clearly water is leaking into grey water tank from somewhere. What I can’t figure out is what leak could cause water to enter that holding tank rather than just leak out the bottom of trailer? We towed it across the country two weeks after we bought it and then put it into use. I can accept the blame for a possible frozen line somewhere as this move happened in Dec. But I can’t figure how it could leak only INTO this grey tank without going down the kitchen sink drain. Any suggestions? Hauling it to dealer is not an option right now as we are living in it due to sudden job change.