27.6 to 28.8v would be normal. This represents .3 to .4v over nominal of each cell in the batteries.
Your reading (27.3v) is a little under, but if your batteries are healthy and charged up, the reading sounds acceptable.
For a test you can discharge your batteries using the headlights and after say, 4 hours run the test again. The depleted batteries should cause the charging system to put more voltage out to replenish the batteries.
Your charging system output is low! But I guess if you have no problems...