As described, its a bayonet cover.
That means it has two protrusions in the plastic that slide down two slots in the van gas fitting then turn to lock in place.
I had the same problem.
Grab some pliers and push and turn the cover a bit at a time until you find the point where it releases.
Should add that on mine, that if the hole for the safety chain is at the 3.00 position it will pull out.
At 1.30 and 4.30 it is in the locked position.
Apply the same principle if the gas fitting is installed differently.
On mine it can get very tight. I have put a dob of paint on the plug/cover to show where to align it to the socket. As Ineeda said, push in hard and rotate it. Hold it with a tea towell or cloth if needed. You should able to feel when it is aligned correctly. Have a look at the gas hose so you know what needs aligning.
Don't forget that a bayonet locking cap will normally only turn one way. Trying to force it the wrong way with pliers or a multi-grip may tear off the lugs and that of course will mean that it will no longer lock back on when want to replace it.
