When you are using a Hydrometer you are not testing a Battery, You are testing a CELL.
12 volt batteries have six cells
Six volt pairs,, also have six cells, 3 in each battery, and thus are a 12 volt battery when installed (I know, confusing) which is why I say there are no six volt batteries IN Rv's (True there are six volt cubes, but they are paired to make 12).
And when using a hydrometer, each 2 volt cell is ... Well that is what the instructions are for, 2 volt cells. You just test .. all of them.