They may have had sealer applied before they were inserted through the shell. Since you have the hinge off, take two nuts and install them on the screw shaft and tighten them against each other. (a jam nut arrangement). Once they are tight then you should be able to break the screw loose by putting pressure with a wrench an the outer nut and turn screw the back into the trailer.
I just pulled the hinges off a Scamp
and put Butyl tape behind the hinges before installing stainless acorn nuts because the hinges had not been sealed.