Pump on AC only

[Cab n Fever]

Howdy,

We have a 2001 Coleman Niagra, that has a pump that works just fine when it is plugged into the house, but when we go to battery power, the pump just makes a quieter humming noise, and there aint nothing coming out of the faucet. In both cases, we are using the water in the tank, and not the city water hookup, and I don't think it's a priming situation, because as soon as we plug it in - it works just fine. Any ideas? Thanks!