help/clarification on the "no gravity water fill"

[Mark N.]

I've searched the forums about filling from an external jug or tank of water when you need to add a few gallons while boondocking. So as I read it, (I have a '13 3400 rl) I can hook a short hose from the jug to my winterize fill, turn the selector knob to fill and turn the pump on. I've missed something. This doesn't work on my Monty. It pressurizes then the pump stops. Is it the city fill as opposed to the winterize port I should be pumping through? There must be something simple I am missing here...