Message center reads Battery High Voltage and Check engine

My 2013 Winnebago tour freight liner chassis message center is reading check engine and changes to Battery High Voltage . Gauge for battery voltage is almost at 18 ? I drove it for 8 miles turned it off for 15 minutes restarted and same message ? Drove 5 miles Battery High Volts went away and battery gauge dropped to 14 volts and check engine light went off ? Any ideas ? Thanks