Installing Outlet for Welder; Possible Dual Use for RV
I appreciate your effort to try and combine outlets and wiring but it's usually pretty hard to do. Your welder will need a 40 Amp breaker, two hots and a ground. Your future RV would probably need at least a 20 Amp breaker, one hot, a neutral and a ground. Weather proof, water tight outlets and cover plates can run into a lot of money so I'd think about installing your welder outlet in the garage. Put a longer cord on your welder or get longer ground and electrode cables so you can weld outside. If and when you get your RV, then deal with the outside outlet. If you run 3/4&quot; conduit and THHN for the welder circuit, you could add the new RV circuit in the conduit and extend it outside with your weatherproof/watertight fittings. If you're planning on running cable, you'd need two separate runs.
As far as testing your welder, A Megger test would be the best for verifying the insulation is good. You could also remove the access panels and clean everything up and check all the connections. Plug it in and strike a small arc at each setting.