There are so many possibilities here, that anything is just a guess. With Zero information to reference, that is the best anybody can do. Your symptoms could be:
A bad level gage (the tank is actually empty)
A bad pick-up (if the tank is a top suction)
A bad suction line (if the pump is above the potable tank)
A bad pump (that is letting air into the system)
A Word about vocabulary.
Holding Tank - Where mixed black and gray waste are stored
Black Tank - Where black waste is stored
Gray Tank - Where gray waste is stored
Potable or Water tank - Where fresh (drinkable even if you don't) water is stored
Matt