Your black tank is still higher than the dump, so it will dump the tank ok, just not empty the hose. Keep the black closed while you are camping. A day or two before you leave, shut the gray water valve off and build up a little gray. Then when you are getting ready to leave dump the black first. When it stops, then open the grey and dump it to wash out the hose. Then when it stops flowing you can massage it to get the rest of the gray out and you're good to go.
RV Sewer Hose Support by Level-Trek - PPL Motor Homes
These work good sometimes also.