Fresh water filler cap

When locked the cap should turn freely, when unlocked it should 'lock up' and then twist off as any normal cap. Is it 'locking up' when you turn the key anti-clockwise?

In my experience on both our Trident & Nuevo you had to hold the cap (to keep it in one position) as you turn the key and likewise for locking... 
exactly the same principle as fuel caps on todays vehicles.

Thank you both for your help. As this is all new to me I wasn't sure exactly how it was supposed to work and that lack of experience coupled with the fact that the cap probably hadn't been removed for some time meant that I thought I'd done something terribly silly. I now know that I did do something silly, I assumed that the cap was locked so I must have been locking it and then trying to remove it. Never mind at least I know now and I didn't have the problem on a site with a queue of people behind waiting to fill their tanks.

Thanks again for the advice.

Triggy

Yes: insert the key; hold the cap steady; turn the key through 90 degrees; then twist off the cap anticlockwise.
Fitting is the reverse, obviously.

You may wish to tie some string to the cord, and attach the other end to a suitable point inside the cab (perhaps near the seat belt base). Then when filling, leave the key in the cap; put the cap on the seat or elsewhere suitable; fill up - and replace the cap and key (which is thus always inside the van). You won't lose key or cap, nor drive off without replacing the cap.

Memories!!!!!!!!!!!1