Ice house battery charger alarm

I have a 2016 ice castle with a new battery installed, got everything ready with the main power unplugged, running lights and heat on just the battery. After about 10-15 min I get the red-green lights in sequence and an alarm about every 15 sec. The manual says it’s either low battery or fault; I hooked up a different battery charger and it said the battery was full— any ideas? Faulty alarm? The charger on the ice castle seems like it’s working as after I put the other charger on it indicted a fully charged battery but what gives with the alarm?

**after the furnace and radio was turned off and just the lights on, the charger showed a steady green light and no alarm