Hello,
If you get water flowing into the bowl with the head intake (under the port settee) closed, the joker valve oin the head is bad. THat allows the
holding tank to backflow into the bowl. If you head is fairly new, or is a good one, you can buy a rebuild kit. The work is not that hard, but is not very pleasant. If your head is old, just buy a new one and be done with it.
I bought a 1986 N 28 a few years ago. I had to replace the head because it was shot. Not a difficult job.
Anyway, enjoy your new boat. I believe that Newport boats are an excellent value today.
Barry