Reefer fuel cap

Gripping outside threads tightly with pliers won't help you loosen them. Try using a band wrench or something like it, you'll have better luck.

 

Find a old fashioned spanner. The one with the adjustable jaws like 3 inches gap wide open. And a metal handle like 3 feet or more long. Find a 2 inch lead pipe about 4 foot to slide into handle to make it 5 plus feet of lever.

Adjust jaws onto the reefer cap, slide bar on and then carefully loosen. Steady as she goes. She'll come up.

Now if you think you got king kong who wont shut up, then squeak that reefer fuel cap shut and hide tool and watch him work himself all out on it.

 

Screwdriver and a hammer

 

I was going to suggest that but, well, he's a reefer driver and probably doesn't have those on board. Purchasing a band wrench is cheaper than a hammer AND screwdriver...LoL

 

Put some anti seize or oil the threads will help you out too