Any good / easy way to find power loss on 12V house side?

If it was me I would see how much current is being drawn from the batteries. With only the fridge and the various detectors on I would expect less than 1 amp drain. If more, I would start removing fuses and see where the current is going.

Without an ammeter you could remove all the fuses and see if the problem persists. If not, replace fuses one at a time and see when the problem returns.