One more question, but first some background .....
My tank is configured with two separate dip top-mounted tubes and discharges that lead to (a) a manual Whale diaphragm pump and (b) a deck pumpout fitting.
In this case, my mechanic is pushing me to put a ball valve between the tank and the diaphragm pump to improve suction and protect the manual pump. I haven't seen this approach recommended anywhere else under the sun, although it doesn't strike me as a terrible idea. Thoughts?