12v, as in 12.0v, is a fairly low for a battery, only about 50% charged. But I doubt if a rattle has anything to do with batteries.
A loud click-click-click when starting the genset would suggest insufficient voltage, but that's not what what would call a "rattle". And the jacks just run slow or not at all if the voltage is too low.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC