I have the Vision PRO kamado but never heard of a PRO III series?? Regardless, I do achieve 250* all day by slowly working the fire up and as soon as I get to about 180* or so I close the bottom right vent all the way and put the bottom left vent on 1. Then I put the top vent on 1. If it's still climbing steadily I drop the bottom left and top vent to .5(just a crack). I usually achieve 250* in about half hour or so and that's it other than checking periodically. I taped up the bottom drawer like yourself and untaped it for testing purposes (just to see if there is any temp fluctuation) with absolutely no difference in temp so I don't touch it at all.
One other thing I can think of is to check the felt gaskets around the ash drawer because I don't have any leaks around that area and my felt gaskets still look really good even after reaching about 1000* or so at times.
