Final update...
At 9 hours 15 minutes, my Thermoworks Alarm went off saying the temp had dropped to 200 degrees. The temp was as high as 400 at one point. It steadied around 320 for a long time, after I shut down the vents.
Had I managed the air flow properly, 10+ hours of 250 degree smoking could have been achieved. Now I know what to do, and will hit my targets much closer next time.
Keep in mind fuel was never added to the fire after we got it going.
And this was all with the Weber Charcoal Baskets. So it seems you do not have to spend $$$ on gadgets to do a low and slow cook.