Temperature - Vent Questions

You might try a bigger fire to start with less oxygen. These long cooks also see the firebox filled well into the fire ring.  I never like to blame the charcoal, but trying different brands doesn't hurt until you find something you like.

An other ideas or is it what it is?

i have two large eggs, vents settinings are different with each but you should quickly learn what works with yours. things thaty change the rules though are an egg not used in a while that picks up moisture in the ceramic, not puttining big lump directly on the grate (small lump on the grate blocks the holes), water pans with water need a bigger fire to keep the egg hot as too much energy goes into evaporating the water, too big of an indirect settup witll force heat down and block heat flow(this doesnt seem possible til you see a 1000 degree fire below a 16 inch pizza stone and a 200 degree dome temp), meat probe getting a fake reading because its too close to the cold meat(both grate probe and dome gage can give this fake reading so probe placement is key) im sure theres other reasons but if your burning a firebox of lump that quickly at that low a temp im sure its one of them

regardless of vents settings, a fire that holds an egg at 250 is going to be about the same size as any other fire that holds 250,  all things being equal.  that means if you light large number of spots to try to establish the proverbial "good" fire (what's implied is a better fire), those fires are going to be about the same size all together as if you'd just lit one fire. lighting in one spot like you did is absolutely fine for a low and slow.

regardless, the airflow thru the egg will be about the same, no noticeable difference. so your vent settings indicate a problem.

something is obviously 'wrong' with your case.

thermometers could be reading low, but you said, your maverick is brand new.  if the temps you refer to are grate temps, that could be the issue.  we say 250 meaning dome temp, not grate.  and there can be a 30-40 degree difference between the two (just the way it is).  if your grate temp was 250, you could really be closer to 300 dome

your egg is WAY open.  i could be at 400 with those temps, and fresh fuel.

royal oak is a good medium lump, not too light, not too dense. it was my go-to lump when it was more available around here, and there's no way it is just going to burn that much in 9 hours unless the fire is too hot, or our definitions of 'full ' are different.

my egg is always full, topping off each time, up into the fire ring or at least as high as the ring, with a mound in the middle.  that will go for a heckuva long time.

you didn't say what temp you took the roast to, or how big it was. that might help us understand if your temps were accurate too.

@ robynybbq- If I recall, this is not the first time you have posted about temperature/vent settings and lump consumption.  Given you have tried everything mentioned above and in the past-I'm at a loss.  As stike says-once stable, the air-flow is what determines the size of the fire which then results in the thermo readings.  Perhaps reaching out to local Egger's is the way to move forward on this.  FWIW-