My Safari MH only has one vent pipe to the roof and it's for the black tank only. The gray tank vents from four different short stack pipes with siphon valves that do not go to and through the ceiling. One each for the toilet room sink, bath sink/shower, W/D and kitchen sink.
It is my understanding that it is common to vent the gray this way and it is done on many MHs. It also is common for this system to suck the water out of the trap for the W/D. I took a survey of the MHs in our park and it's about 50/50 for one verses two roof stack pipes.
2005 Safari Cheetah 38PDQ - 2009 Ford Flex
Me (Gatogonow), The Boss (DW), Honey Bunny, Maggie May and Mollie Kay (The Gatos)!