Galley....sounds like you have three dump valves....1 for black, 1 for grey and 1 for galley. Are you opening ALL three?
When you are dumping do ALL the valves feel like they are opening?
Are they cable operated drain valves?
You can fix it yourself......you will have to gain access to galley tank drain valve and repair/replace cable operator. That means removing underbelly OR cutting access window in underbelly where drain valve is located
I'm very handy BUT if it was my rig and this new I would return to dealer and have them repair WITHOUT cutting window. Have/MAKE them remove panel.
Otherwise do it yourself.