been doing this for almost 10 years and it works great. I have the 7 gallon blue square water jugs. I have a 30" long SS roughly 1/4" dia tube hooked to flexible water line to the pump inlet. then I took off the vent cap on the blue jug, drilled a hole in the vent the size of the 30" dip tube, angle cut the bottom of the dip tube. the extra pump is hard mounted in the pass through with the outlet hard coupled to the trailer tank water line.
normally the cap and vent cap are on the jugs. When I go to fill, I remove the caps, push the dip tube in the vent hole all the way to the bottom, turn on the pump. dip tube stays all the way to the bottom and I pump away.