I finally found where I think the flush line enters the black water tank on my 3170DS. It appears that it enters at the top of the black water tank by going in under the wood partition that separates the undersink cabinet from the cabinet to the left of the undersink cabinet in the bathroom, behind some removable wood trim and some not so easily removable wood trim at the back of both cabinets, underneath a pvc drain pipe, electrical lines, and hot & cold water feed lines for the bathroom sink/tub that run horizontally along the floor at the back of the cabinets behind the wood trim. The fitting does not appear to be accessible without removing the bathroom sink cabinet or cutting a large hole in the partition to the cabinet, cutting out the drain pipe that runs along the wall and moving the electric and water lines out of the way, then I might get a glimpse of the fitting.
That's why I wanted to know if the fitting was actually there before I went and removed all of these things to try to gain access to the fitting to remove and clear the blockage.
I guess it doesn't matter now since I again connected an airline at 60 psi to the black tank flush line and this time it blew out whatever has been clogging this line for the past few months.