I think you need to investigate the issue further. whether voltage regulation or a failing alternator or failing connections between alternator and house batteries, Something Ain't right.
If the engine AGM battery is only used as a starting battery and not deeply cycled then I'd not really worry abot the voltage it sees.
Seems all vehicle voltage regulation is about not OVercharging, and thinking 13.7v is more or less, just fine and dandy, always.
I love overriding that misconception at the twist of a dial.
