@ia_camper is correct. If you’ve emptied the grey tank and there’s still an issue, it’s a clog.
1. Check the drain cover or screen
Many RV showers have a small strainer under the drain cover. Hair often collects right there. Try removing the cover and pulling out anything you can see.
2. Use a plastic drain snake
A thin plastic drain snake works well for this. Slide it down the drain slowly and pull it back out. They are designed to grab hair and gunk without damaging the pipes.