Converting NEMA 14-30R to TT-30R?

Chris Bryant wrote:
As long as the 14-30 was wired correctly, that adapter will work fine. It only uses one leg of the outlet.

But if it WAS NOT WIRED CORRECTLY!!!

This electrician didn't install the Receptacle you need. What if s/he didn't set up the correct WIRING? The right BREAKER? The right VOLTAGE?

Was a new Branch Circuit, with a new Breaker, installed? If so, can you identify that Breaker? If it's a "double" meaning it uses two positions on the panel like this Then I have to say:
DO NOT TAKE A CHANCE!!! It's already the wrong receptacle and unless you asked for higher capacity service, the wrong wiring (Too many conductors as noted above. Doesn't hurt anything if done correctly but more expense.), and if it's wired wrong besides, what Chris' quote above means, you could end up feeding 240 Volts into a coach set up for 120 Volts and the consequences will be severe! Loads of Damage!
OK, let's say s/he installed a single breaker like this: You'd only get the 120 Volts you asked for, but you aren't out of the woods yet. Somebody who didn't provide what you asked for, may have also mis-wired the 14-30R, so now your adapted outlet won't be wired right for TT-30.
I really hope you will call the electrician back and ask for a walk through of your new outlet installation and circuitry. The Breaker that was installed. The wiring that was installed. How the new, correct, TT-30 is installed, showing Voltage and Correct Polarity. You need to be sure Voltage is really 120, AND that the Polarity of the Receptacle is right. If not, you can end up with no Safety Ground and/or what we call "Hot Skin" here Hot and Neutral are swapped. This means 120 Volts can go to things you touch and step on around your coach.
Get a 30 ==> 20 adapter and one of these outlet testers With that, you can verify that your pedestal, home garage receptacle, and anybody's campground pedestal, is properly wired for your 120 Volt coach. It should show a certain pattern in its lights. If not, DON'T PLUG IN.