Your pump pickup is probably in the side of the tank, maybe as far down as possible to the bottom, or maybe not!!!
If your FW tank drain is in the bottom of the tank, tee into it and move the pump pickup to that point. Many trailers have this problem. Made worse by small tanks, so you notice it sooner!! Also, not level or slight tilt to the pickup side.
You, or service place, may still need to slice or open up the bottom to do this, but will have to for anything around the pickup. Others have added a weighted hose that extends into the tank to try to get as much water as poss. I think, if your drain is easily accessible, a tee in is the best.
I have done this on previous trailers, and have never had a problem with the "elements" affecting the water supply. Camped in 20 deg temp without freezing. If worried about freezing, foam it in!!
