Replacement holding tank drain valve on Flagstaff 228MAC

[AR_Vern]

I recently purchased a 2007 Flagstaff 228 MAC pop-up. The fresh water tank drain valve is missing, and I'm having a little trouble identifying the size for a replacement. The holding tank is under the camper with the valve just in front of the tire on the drivers side. The gentleman I bought it from had purchased one of the the universal aftermarket valves that goes from 1/2" down to 3/8". The diameter on that valve is too small. I tried a 3/4in pvc plug but it is a little too big. A rough measurement with a caliper shows the distance inside the female threads is just over .7 inches. Does anyone know how I can identify/find the actual part or can someone help me identify the size so I can potentially find an aftermarket plug from a hardware store?

Thanks!