Water running outside - overflow?

Hi

We've been getting ready for a weekend away and i put water in the tank but when we tested the water pump very little water is coming out of the tap, but water is coming out of an overflow (?) or similar outlet on the same side as the inlet.  any ideas?

thanks for any help.

Is it coming out of the water heater external flue, have you put the drain plug back in the heater after draining down for winter. Any overflow would be underneath the van or is that what you mean? I think a bit more info is needed.

Hi,
My van has the hot water drain tap under the bed next to the cooker, lift the bed settee back, pull out the large plastic plug and you can see the tap underneath, the yellow lever should be level for closed and you lift it to vertical to drain,
Chris