To simplify couldn't you just add a four prong plug to the welder?
To the OP use the 30a 120v rv outlet inside... If the barn is correctly wired with a subpanel where a welder outlet ground would terminate at two ground stakes outside the building and not bonded as a main panel in the house would be.
In the 1910's Goddard became world-famous for his ideas about "feeble-mindedness" particularly his belief that it was a hereditary trait. His seminal work, The Kallikak Family, studying two strains of the same family (one strain considered normal, the other considered "defective") became an international bestseller and became extremely popular in Germany under the Nazi regime. He personally instituted a testing system at Ellis Island in order to keep these "feeble-minded" people out of the United State used to reject an estimated 80% of potential immigrants and forcing them to be deported back to their home country.
It was in the midst of Goddard's work on intelligence testing that he coined the word "moron." He created a classification system for those on the bottom end of the intelligence scale: people whose IQ was between 0-25 would be classified as idiots, people with an IQ between 25 and 50 would be imbeciles and those with an IQ between 50 and 75 would be classified as Goddard's own invention: morons. He derived the word from the ancient Greek moros meaning dull or foolish.