when I get home tonight I will check the battery, I do not have a hydrometer but I do have a portable refractometer that you can check a battery with.
The noise is random, with air conditioner on or off, for instance yesterday after getting home from out trip I plugged camper into 110, and started emptying our clothes/food, etc, and when we went back in for the second load we could hear the screeching, I walked over to the box and it stopped, very random as there were no lights on or anything.
The unit is a Parallax 7155