All three tanks, black, gray and galley are the same size...38 USG. I think the tank layout is the same on all the Rockwood/Flagstaff fivers except with the half bath. The black and gray tanks are side by side (running north/south) under the basement. The fresh tank tank is centered over the axles (oriented east/west) and the galley tank just behind the axles (also oriented east/west). All tanks are mounted between the frame rails at the same elevation.
The piped mod that dunnc has done and posted pictures of will also equalize the levels in the two gray tanks when both valves are open and the black tank valve closed (of course).
He's done a great job on this mod...I can't think of any other practical way to tie the two together.
Nights camped in 2013 - 55, 2014 - 105, 2015 - 63