Probably low battery. Does the beep start 30 seconds, 30 minutes or 30 hours after shore power disconnected? And what is running inside? What do your battery cables look like at the battery? Have you pulled the caps and checked the battery water?
Is your inverter also a charger?
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2003 Dolphin LX 6355 w/ W22 chassis; 8.1L gas & Allison 1000
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