Looked at the floor plan and it appears there is a rear kitchen. So my guess would be there is a separate gray tank at the rear of the coach for the kitchen. Since it is so far towards the rear, it is impossible to plumb the drain to have continuous 1/8" per foot of slope and not interfere with the axles to tie in with the other drains, I would assume there is a separate pull handle for that rear tank under the skirt metal in the back. Look for the separate termination too.
Chris & Maria
2012 36RL, Full Body Paint, Dual Awning, Dual a/c, Splendide Combo, Fisher Paykel dishwasher, 6.5kW Onan, Goodyear G614's. 2015 F550 6.7l, 4X4, Lariat, CC, 176" WB, Long Box, B&W Companion