What happens if you plug a 50 Amp RV into 22V welder plug

Sorry this happened to you. I don't have any words of wisdom except to begin checking everything with your multimeter and add up the damage. Stay in touch and let us know. To my knowledge the combined neutral & ground together is no longer recommended for new installations like Dryers and welders but someone who has bought a new welder recently may be able to confirm this. If you had a four wire welder outlet, an RV would run off it with the appropriate adapter as the ground & neutral are separate.