Put a meter on the battery itself. If under 12volt then either shore power or generator on and check again. Should be close to 13.4 volts. Let run for 30 minutes . Turn off shore/generator which ever power and check voltage, if still at or below 12 volt, then bad or very discharge batter. If battery voltage is below 12 or less they will not charge. They must be charge a different way to see if bad. Watch this and try it. Works if your battery has any life left in it. If you gets voltage then put on a charge at no more than 10 amps and let charge.
https://www.youtube.com/watch?v=gMPuzmhYCQc