I'd kindly ask them to limit the area they park on and aside from that, if the rent is "right" I wouldn't get too worked up about it.

I'm having a hard time envisioning how 10-15 cars can wheel pack 2 acres thoroughly enough that nothing will grow on it.    15 cars should be able to easily park on a quarter of an acre.    Is the path from the road access to the parking area 3/4 of a mile long or something?