My opinion is most thieves are not very smart and if they want to put Diesel in their cars/trucks then good for them. I do not think that most of the people that camp will steel fuel from your truck. If they wanted to steel there is a lot more valuable items around the camp site then fuel in your truck.
I did have a locking gas cap once on one of my vehicles a long time ago nothing but a big problem. Every time my wife would try to fill up she would forget to unlock the cap. She would have to put the fuel hose back into the slot on the pump and go to the car, get keys and then unlock cap.
Just my $0. 02