A 27 foot would be an Overlander. The bath is in the rear. Our 74 Overlander had a HUGE 10 gallon grey tank. If someone had tank work done then adding one at that time would have been possible. You may not have a gauge to monitor it since it was not standard equipment. I would ask what size the tanks, both black and grey are. You may need to ask a member what size black they have in the same model/year coach to see if the total gallons that can be held has increased. The biggest issue is weight. The frame in 71 was not sized for a ton of additional weight back there. If they added the grey without decreasing the black you could have a problem. Of course if you can ALLWAYS tow it with the grey tank empty the weight is not an issue.
Note: all caps=sarcasm
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato