Howdy,I agree with having voltage drop, but it's not possible to drop the frequency over a wire distance no matter how much resistance there is. It will be the same frequency no matter the voltage at either end.
Unless my understanding of how alternating current is way off and my google fu is failing me that's how it is.
What I am saying is;
The MEP-803A is mechanical. If your average running is around a 60% load, adjust accordingly. A nice 120/240 60Hz. So that means a unit with no load might be sitting differently on the gauges.
A no load MEP-803A for me would be hanging around 61-62Hz with no load.
I would take all the readings from where the power is being used. The biggest load being the A/C unit. Measure at the A/C, not 600ft at the generator.