Can I plug my welder into an RV outlet?

No you cannot plug your RV into a plug designed for a welder. The plug for the welder is 220 volts. If you figured out some way to rig a plug to fit the welder plug and it was not configured right you would immediately blow up all of your RVs electrical appliances.

Will a 50 amp camper run on 30 amps?

Yes! With an adapter, you can plug a 50-amp RV cord into a 30-amp power pedestal at a campground. However, note that without a 50-amp receptacle, you wont be operating at full power, so there will likely be some limitations on how many appliances you can run at once.