Had a similar situation with my 2018 FR3 29DS. Finally discovered that the clamp securing the flexible drain line from the kitchen sink to the ABS drain line to the gray water tank had come loose. I had to pull out the oven/cook top to get to it. The water would always appear after retracting the main slide with the kitchen sink. Not sure of your layout, but it's worth looking into.
The way I see it, you can either work for a living or you can fly airplanes. Me, I'd rather fly. Len Morgan
2018 FR3 29DS
TOAD - 2002 Toyota Tacoma 4WD