Wow...can't say as I've ever seen a setup quite like that. I can see how you would be confused. I would say just from a common sense perspective (that not being very common sometimes!), the top port would be city hookup (as wneise noted), and the one on the side left is black tand flush. I would base this on the fact that when flushing the black tank, you would not want to risk any "stuff" coming back down out of that port. Also, it would make sense that the city be on the top so the hose is hanging straight down to minimize any chance of kinking the hose.
Ed and Sharon
2010 Wildcat 28RKBS
2011 Ford F-250 XLT, 6.7L PSD
Ride-Rite Airbags and Airlift wireless remote compressor...nice!
I thought I was wrong once, but I was wrong!