I hope someone with a better answer chimes in, but I would check for a kink or obstruction n the pipe under the sink or you may need to vent the hose, there was a thread about adding a vent to the garden hose drain to help it drain - not sure if the link will work (if not search grey water vent, this should come up):
Grey water set up
Depending on where you camp, you may need to collect your grey water as some areas and campgrounds to not want/allow you to dump the grey on the ground. Many people use a 5 gallon aquatainer or bucket to collect the gray water and then dump it at the campground dump station or at the bathhouse.
Hope you find the issue with your drain - and many happy camping trips.
HTT: "EscapeII" 2016 Shamrock 23WS (current)
PUP: "Escape" 2010 Rockwood HW 277 (gone)
TV: "Bruce" 2011 Dodge Durango V8 Hemi Crew
Just us gals (me, Sis and our daughters)
Traveled 19520 miles and 209 nights in 7 years and counting