do you have a charcoal basket? i used to be able to get about 4-5 hours on a basket. some fluctuations but not too bad
That is exactly what is needed in the SFB of your CharGriller.... a charcoal basket. Cuts down on the need to add charcoal every hour, better air flow because the ash doesn't suffocate the burning charcoal for oxygen... the ash simply falls through the basket onto the charcoal drawer thus allowing the charcoal to burn more efficiently for smoking purposes.
The CG still needs some tending to in maintaining long burn times. If the temp drops like it did on you, to raise the temps quicker, slightly open the SFB's charcoal drawer to allow more oxygen into it. Monitor your main chamber therm and when it gets to 225-250, close the SFB charcoal drawer and use the SFB's intake vent to stabilize the temp in your main chamber. Always worked for me if I wasn't paying attention and the temp dropped too much and I wanted to raise the temp quicker. Also, give a little shake to the charcoal basket to loosen any ash just in case, every couple of hours.
One other bit of advise, don't use the OEM therm on the upper lid of the main chamber. It sits too high and won't provide you an accurate temp at the food grate level. I placed a therm on my CG on the left side, as shown here:
Here is the charcoal basket I made quite a while ago when I used to do the smoking using my CG ---
I made my charcoal basket with expanded metal and it measures 12"x12"x6" deep.
Here's a closer view of my charcoal basket and angle iron pieces for its support:
Angle iron in the SFB for support of the basket. No drilling required and the charcoal drawer can be removed without needing to remove or mess with the charcoal basket:
Charcoal basket seated on the lips of the angle iron in the SFB: