My Salem vanity sink (and shower) drains to the gray tank. I also do the geo method and when I dump, I use the vanity sink to let the gray refill with clean water before dumping (rinsing) again, so I know it does not go to the black tank. Don't know how different the plumbing is between a Salem 26TBUD and a Salem 292FKDS, but there ya go. Good luck!
2015 XLR Hyperlite 30HFS5 (mods being performed regularly)
2009 Salem LA 292fkds (gone)
Nights- ('12)23 ('13)23 ('14)15 ('15)31 ('16)25
2014 Harley Davidson FLHX (XLR cargo)
2014 VW Jetta TDI (diesel sipper, camping support vehicle)
2011 Ram 2500 CC 4X4 CTD, B&W Companion (toy hauler hauler)