Not sure about the WindSport...however, in our Axis the bathroom sink does empty into the Black Water tank and has in our previous RV's ... It appears that is by design to make sure there is plenty of water flow through that tank.
Ron & Kay n KC too !
2016 Axis 24.1 E-450 6 spd.
'KC's Big Box'
Jeep Wrangler TJ
or 6x10 enclosed motorcycle trailer
My 2014 hurricane 32a (cousin of Windsport) does drain bath sink into the black tank... Other models may vary - but a quick peek under the sink will show you which direction the drain pipe goes...
In my layout - no way for the bathroom sink to get to the gray tank without a pipe crossing the hallway that I would need to step over - or a pump as it will need to go uphill from under the floor to the gray tank (which is directly under the shower - up from the floor)