Assuming the generator uses battery power from the coach batteries to start, and there's no power to the gen, with the 50A power off, check the salesman switch (coach battery enable/disable) first. Your system might require it to be enabled for starting power to get to the generator.
If that is not the problem, then turn off the 50A shore power, then recheck the coach batteries SoC after turning on a few 12VDC lights or the water pump for a few minutes, to remove surface charge from them. It's probably low/bad coach battteries.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)