Our bathroom sink/shower drain into the same holding tank, separate from the kitchen.
Could just be construction crud in the shower drain that's plugged, along the way to the tank.
Could be just the tank is full and the sink drain is almost full.
Pull the valve to drain the bathroom grey tank and see if that helps. Good luck!
wife to David, mom to the 3Ms
2006 Dodge RAM 2500 TRX4 Cummins
2011 Cedar Creek Silverback 35K