Well where is the water tank?
I will describe many drains
on two trailers I had the drain was directly under the tank, it was a pipe with a cap, no valve. You removed the cap and stand back.
On my motor home it's a valve inside the compartment with the water tank. Look for a tube, pipe or hose that extends down below the tank and go up from there.
There is also a plug I can pull out of the tank,, I added a valve there.
Normally the drain should be in the BOTTOM of the tank.. but some idiot thought 1" abvoe the bottom was a good place for mine,, As F. Flintstone woudl say "Rozzin, Frozxzing, Friggin, Fraggot"
Home is where I park it!