+1 on tru tel gauges, also maybe move the 4" pipe higher in the firebox chamber barrel, your trapping alot of heat in the top 3" of the barrel, and as u suggested more air to fire, but dont do any changes till you get good gauges theres is a brethren here who sells them good prices and grest people, 5-0 bbq out of maryland, bbqequipmentstore.com ive used them multiple times. 1 more thing, how do you control your exhuast draft? is there something in the lid of the cook chamber?
charbroil offset, custom trailer pit, 46" vermont castings gasser, 18" kettle, big poppa smokers engineered drum kit smoker than was never picked up = NEW TOY!
and last but not least: a never ending 20' comp trailer project