When I installed a new Dometic tank a few years ago it had a pretty neat design for the diptube. They used a 30 degree elbow near the bottom and then put a short length of pipe with a large chamfer on it . This allowed more open pipe area and still drained the tank to a reasonably low level without plugging.
I'll see if I can dig up a picture.
A square cut tube close to the bottom may plug easily, or leave "solids"
in the tank.