Power Issues - Generator not starting - brand new batteries

If it is a diesel, I assume that it runs off the fuel in your "gas" tank. Usually there is a level of fuel in the tank that a generator will not run when below that level. Sometimes a 1/4 tank, sometimes more or less. Make sure your tank is past a half full. Onan generators also will not run if the generator's oil level is low. Check that if there is sufficient fuel in your tank.