1. I'm using hickory chunks. but getting very little smoke.
2. with vent all the way open and gas all the way down, can't keep it below 245*
First the factory installed thermometer is more than likely way off you need to check it with a thermometer you know is accurate.
What you want is Thin Blue Smoke (TBS) thin and blue or no smoke and just the smell of the wood burning. Personally I would suggest a tube smoker from A-Maze-N Products then you'll have great smoke all the time without the hassle of chips chunks and trying to get them to burn correctly
Did you check that thermo for accuracy?
I think you are going to have to either have water or sand in the water pan to maintain lower temps
Put some water in the water pan and see what the temps do.
As for testing the accuracy of the ET put some water in a pot on the stove and get it boiling then with a gloved hand or pot holder place the end of the probe into the water and see what it reads see how close to 212 it is. Once you know it's close get a potato and push the probe through it so that an inch or two sticks out the other side then place it on the grate in the smoker and see what it reads. Adjust the smoker to get to 225 on the ET then see what the factory thermo reads so you'll know in the future