The quick answer is NO HARM...
It does seem however to drop down rather fast for that short of time.
My two 12VDC GP24 Interstate vatteries have been sitting outside in the battery box totally dosconnected since late October last year and I just checked them a couple weeks back and they read 12.6VDC. They were on the converter/charger about two months back for three or four days.
consider this chart showing DC Voltage verses state of charge of deep cycle batteries...

You should not ever let a deep cycle battery discharge below the 0% SOC or 10.5VDC or it may do damage to the battery.
Most of us like to not let the deep cycle batteries drop below 50% SOC or 12.0VDC.
Roy Ken