Having driven the roadster across the US several times relying on campsites I know why that was the original choice for the 2010 UMC, but I don’t know if any level 2 charger that can take advantage of 40A 120v. It’s not 2010 anymore, Tesla has super chargers now and I don’t drive a motorhome that requires a 120 and 240 hookup, an adaptor is $50, what’s the extra conductor cost? I don’t know the answer. The only reason I would run a 3 core would to be able to put a panel or load center in place of the outlet at a later date. But to have a 14-50 solely for an exclusive 240v outlet makes no sense any more. The 6-50 is a much better outlet for that application.
Again at some point you have to trust the installer, if you keep questioning his decisions then don’t be upset if you see the costs of installation increasing. The issue annoys me because the information is out there, like many I did my own installation and pulled a permit. If you tell the installer you want a 14-50 don’t question what they do, try telling them you want a pure 240v 40a outlet and see what they suggest, it won’t be a 14-50.