It's not common and the chances of the office saying "Yes our 50A plug is the cheater type" is between zero and none.
I would think that the bigger problem is that regardless of your reservation they don't have a 50A site when you arrive and/or recharging of your car is not allowed.
But consider:
1. Make/buy a simple tester which will tell you it's correctly wired and test the site as soon as you arrive.
2. Stealth mode: Install a female 50A plug in the RV that is not so obvious for charging the car and carry an extension cord.
3. And the MOST IMPORTANT: Have a backup plan to get the car charged.
