VW Trooper water tanks

Are you sure it's blocked? These valves are what is known as 1/4 turn valves, ie, you move them 90 degress to open or close them. Most, however have no end stops so it is possible to turn them beyond 90 degrees.

I'd try this, put some water in the tank so you actually know there's some in there.

Open the valve and see if anything comes out. If not at least try and turn it the other way, if it lets you.

If that fails I'd remove the tap altogether and see if the tank drains then

If not, then it is blocked, this would be unusual and I'd certainly want to find why its blocked and wouldn't consider drinking any of the water until the problem is solved

If it does, then the tap is faulty.

Taps are cheap and easy to get

Good idea.
There are several thoughts on here re fresh water in tanks.
My personal opinion is that we never use this water for drinking.
Several reasons....
Our local tap water is not nice anyway and that's what would fill it pre trip.
By never drinking it it doesn't matter to much what water you put in it, good as possible of course.
Medically I have to drink a lot of water so we carry 6x2 ltr bottles off sparkling water, much nicer to drink than plain.
When you boil sparkling water to make tea, the bubbles disappear and you get normal water.