For our buses, the fill port and cap location is behind a locking door flap. But on our Freightliner ambulance, the fuel cap is exposed on the tank beneath the steps. We had to replace the cap with a locking one, to meet some convention show requirements in Austin, and we paid about $100 for a nice Stant locking cap. And even though it's a top-quality cap, it's a bit scary to use. The locking mechanism is a toothed cam and if you lock it, then someone (intentionally or accidentally) tries to unscrew the cap...or if they fire marshal tightens to make sure it's secure, the teeth grab the inside of the fill pipe and it's then very hard to unlock. Although I've not yet broken the key, it sure makes me cringe. If it weren't for the show requirements at some events, I'd skip the locking cap.
On a side note, a regular 2" threaded pipe cap works fine. But most "real" caps have meltable sections, in case of a fire I guess...