Going to need more info.
Was the gen set turning over at all ? Turning over but too slowly to start?
Was the coach on shore power or in storage ?
Does the gen set start from the coach batteries , or the chassis batteries ?
Because, if you coach is like mine ; gen set starts from the coach batteries; and their state of charge was too low to start the gen set, then starting the coach would supply full chassis battery and alternator power to start the gen set, through the coaches all battery charging system.