Put in the proper RV-style 240-volt receptacle in place of your current welder plug. It's a NEMA 14-50R
. Accept no substitutes.
MAKE SURE you tell the electrician you want all four wires in place, even if you have to pull new wires into the box from the panel! Tell him or her that it HAS to be a full, correct, 50-amp service. That includes a ganged 50-amp double breaker
L1, L2, neutral, and ground. ALL FOUR!!!
Then, buy the RV adapter from post #2 above to get your 30-amp RV service.
Then, replace your welder's plug to match the receptacle.
Since I haven't used my welder in a long time, that last part I have yet to accomplish, but someday, when I get a Round Tuit