For once I found something useful on facebook.
"Evaporating one pound (about a pint) of water absorbs 970 BTUs of heat and drops the temperature of the air it evaporates into. 12,000 BTU's equals one "ton" of air conditioning. Most RV AC units are either 13,500 or 15,000 BTU's per hour.The problem is that water evaporative cooling works best in hot dry air, and hardly does anything in hot humid air. So they are great in the arid areas like the southwest, but not so good in the humid areas like the southeast."
Yesterday the cooler used about 13 lbs of water over a time frame of 8 hours--so it removed about 12000 btu's or as much heat as running the roof air for nearly an hour.
I had re-positioned it to draw hot air from the glass of the window beside the dinette.