UPDATE to 240v 30amp recepticle fries another one!

I just heard from my friend who had his 30amp RV receptacle wired with 240v instead of 120v. The electrician fixed the problem when he wasn't home but left him a message saying so. My friend had visiting friends with a class C plug into the same outlet. Guess what? Funny things started happening to his rig. They checked the outlet again and found the neutral and hot reversed. I am now thinking this electrician, and I use the term loosely, doesn't have a clue! And yes, I had told the owner he should use a voltmeter to check before plugging in but evidently you can't teach an old dog new tricks.
Dave