water pump issue HELP!!

I would bet a loose/leaky connection between the tank and pump. When the tank is full, there's enough pressure to keep it from sucking air, but when it gets low, the pump actually sucks in some air.

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

Could be a venting issue. No air in, no water out.

We will double check but im sure its full we tried everything.

Probably going to have to get under and drop the tank.
BTW are you positive the tank is 3/4 full when this happens? I think before i went this far, i would refil the tank using a 5 gallon container and see how much water i could put in. It might simply be an empty tank

How do we do that?

My guess the pickup hose in the tank has broken off. See if you can pull it out and measure it.