When the battery on my camper gets low (if the camper is not plugged in for a while), my power converter will chirp. I can hear it from outside the camper.
You said you have a new battery, but is it really charged up? Are you sure your have shore power? Maybe a breaker in the shed flipped, and over time, the battery with parasetic draw, simply depleted the battery.
Make sure you on-shore power source is active. Check inside with an appliance plugged in to make sure your camper is getting power. But, sure does sound like the power converter and low battery warning chirp to me.