1986 Jayco Dove (or songbird) tank tap

[JayAce]

I have just bought this poptop, and there is a threaded hole in the under-van water tank. I would like to fill that hole with a tap so that I can wash my hands while driving, or drain the tank.
Where would I buy such a tap? Could I get something from the local hardware store?

Photo of the tank and my attempt at measuring between the threads of the white plug: https://imgur.com/a/nh4nteJ

Thanks