leak under bathroom sink

What you are referring to is the air gap/ back flow preventor for the sink.  A few years ago another podder near us had the same issue.  We went through all the connections and snugged them up a bit more, problem solved.  If that doesn't remedy your problem check the gaskets and o-rings on the air gap, they may not be seated properly or may need replacing (which would probably only be necessary if an owner before you had the same problem and overly tightened the fitting).  Also, your 178 has to be at least manufactured in 2013 or later, if they were made in 2011 we probably would have purchased one.

Thank you Wood River, looks a lot different from our 2011 model.  BTW  what do you use to post your pictures? Thanks

I want to thank you both for posting replies. Wood River Pod, that pic looks like it. It makes sense to be a black tank back flow preventer. It was leaking during the flushing of the black tank.  As I mentioned it seems to be leaking from the square portion. The problem is access is very limited. The device barely protrudes from the wall cavity under the sink. Fortunately it protruded just enough that the water was dripping in the shower base rather than behind the wall.  I assume I will have to remove the sink to replace it.  I dont think I trust cleaning the original or perhaps repairing it. It will have to be replaced. I guess I'll have to contact Forrest River for a replacement. Are there any surprises when removing the sink? Also If I decide to remove the sink completely how do I seal up the hole in the wall? Thanks again for the replies. 

Markie,
Had a similar problem with a leaking valve. I removed it and found that 3 rather large stink bugs had taken up residence in the square section of the valve. Rather easy fix to clean them out and reinstall the valve. I put screening around the openings to keep the bugs out and haven't had any problems since.