I missed who said they had both tank drain & low point drain?
If it was in reference to my post , I should clarify that on my coach; I don't have a freshwater tank drain the way you are alluding too. I do have two sets of drain lines that both happen to be low points under the coach. Each set has tandem red & blue pex line with drain valve. So that is total of 4 pex valves and 4 pex drain lines. I can open all 4 valves and my fresh water will NOT drain. But when I turn on the pump with valves open, it will dump all 70 gallons in less than 5 minutes. Works great and these drains are no where near the freshwater tank. The only way out for water in my tank is through the pump. I guess I could remove the pump and insert my own drain line w/valve before the pump and have both. But to your point, what would be the point