Sounds like time to do a good check of the manual as I don't think of having the pump on to fill the tank and I would guess it is a matter of not having done things in the right order. On my motorhome, I get very low pressure from the tap if I leave the valve in the fill position.
Since the vale is such a simple item, I would not expect a new one to be defective. To do a check of the manual, we would need to know year, make and model of the unit as there are way too many to look through using only the model. Sorry.