very slow kitchen sink drain - brand new Clipper 17BH

[martom]

We just bought our very first RV travel trailer, a 2016 Coachmen Clipper 17BH. Picked it up yesterday! One issue we're having is that the kitchen sink drains very slowly. After the water being on for about 2-3 seconds, the sink starts to fill up. When I shut the water off, it slowly drains. I can see some plumbing glue (foamy beige stuff) when I look down the sink, and it looks kind of sloppy on the outside of the pipe under the sink so wondering if the glue is clogging the pipe. We brought it back to the dealership and they didn't even look at it, just said the sinks always drain slow on these trailers. That doesn't sound right to me. Does anyone know anything about this?

Thanks, we're looking forward to having some help from the forum since we're newbies!