If you are talking about these kind of ADAPTERS, the answer is yes provided your 50 amp outlet has two 120 volt hot lines, a neutral
, and a ground. The adapter will just take one leg of the 240 volt 50 amp outlet, to provide 120 volts to your trailer. It terminates the second leg of the 240 outlet inside the adapter so it does not provide any power to anything.
Wal-mart carries these adapters in their RV section.
Just remember that although you are plugged into a 50 amp outlet, you are still limited to just 30 amps by your travel trailers electrical system.
If your 50 amp welder outlet isn't one with a neutral, then read the following link, where you can read about welder service and 50 amp service on the left hand side tabs: