Yes, that is a vent pipe for the bathroom. It is there to make sure the bathroom sink and shower drain without any problems. Once I started using an odor control for the black tanks, it really cut down on what we smell when the fan is in operation. Really the only smell we get now with the fan on is when the toilet is flushed and drawing large amounts of air from the toilet.
2012 Sandpiper 365SAQ weighing @ 15k, Onan, 2nd air, slide toppers, TST
2013 F350 6.7L Lariat 4x4 CC DRW,Viair 1007, Ride-Rites, Edge CTS, B&W
2001 Chevy 3500CC DRW 4x4, 8.1L, 4.10 gears w/ Detroit TrueTrac
2001 Ford Excursion Limited 4x4 7.3L,V/B Spring Mod, ProComps, Hellwig Swaybar, & other 'Necessities'