Sounds like its about the same as your drum...10 lbs lump and two splits on the fuel for 6 hours...with water...the temp is going down at 6 hours too. If you choose to use water, the water will need to be replenished around the same time as the fuel. Again, if you do use water it is going to be hard in my experience to get 300 out of it. If you don't use water, I wouldn't put that much fuel in there. You will be cooking at 350 or higher based on my skills of controlling that much fuel and the way it spreads. When I go hot, I light the fire opposite the one intake (slide) that I have on mine and it helps in not spreading the fire too fast. You may be able to keep your temps down to 300 with that much fuel, but I have not had that much luck. Cooking around 300 takes more tending for me and what I have learned...a smaller load or fire and more tending or it will get too hot. The vault likes to cook really hot without water.
LBGE, Traeger 075, Pitmaker Vault on LT-1 Trailer