I use whole house wired one.
A while ago I plugged into the 30 amp outlet and the surge protector(Electrical Management System) told me the circuit was not grounded. I informed the campground host and showed him the indicator on the equipment. He said people had been using that spot for weeks and that means there is nothing wrong with the outlet. I said, no it doesn't. It simply means the users were lucky and no one was electrocuted. I moved to a different spot then quietly posted a note on the other box alerting the next user to be sure to test the ground before hooking up.
On several occasions it's alerted me to a low voltage condition that can be harmful to electric motors like the A/C.
For me, it's money well spent.
ed