I don't have a UDS, but a WSM is similar.
If you are lighting up all that charcoal at once, that would certainly turn your meat into charcoal. As mentioned, use the Minion method, so that at any given time, only a small amount of your fuel is burning, and the fire works it's way through the fuel like a slow burning fuse.
I also have to suspect, as others do, that your thermometer is not telling you the real temperature in your cooking chamber -- not even close. That much burning fuel and airflow has got to be making heat well above 300 degrees.