First Pork Butt

You shouldn't have a problem if you fill it to just below the plate setter or a little above the fire box. I did 2 pork butts turbo yesterday ranging from 350 to 400 for 600 hours and I still have a lot left. Now I have an XL, but your medium should be able to handle 12 hours no problem if you put enough lump in it. If you are are going low and slow the lump won't burn as fast either. Leave at least an 1" of air gap between the lump and bottom of the plate setter.

On thermometers, if you can afford Thermoworks I would go that way, but I got my dual gauge on Amazon and like it so far. Here it is.

Ivation Long Range Wireless Dual 2 Probe BBQ Smoker Meat Thermometer Set - Monitor your Grill From up to 300 feet away most of the time. Now Amazon also has a nice pit controller for $139 for your size egg that would help keep temp better and monitor the meat temp at the same time. Lot of options here. Good luck and have fun.

I have gotten 8 hours on a small and 30 on a large, so I expect 12 on a medium should be no problem.  Like Ladeback69 says, fill it to just below your platesetter.  Enjoy your experience.