carbon in tail pipe

[demeat]

hey gang, just came back from myrtle beach n.c .with my camper in tow. it was a 1,440 mi. round trip. a\c crankin', fully loaded truck and camper, i avg'd about 9mpg. since i got the truck new in feb, i put 12k mi. on it. all highway mi. i've been changing the oil religiously before 3k mi. because of my concern with the piston slap issue.  i want to keep it as clean as possible, cause i'm going to keep my baby as long as i can. my concern is at the end of the trip, i noticed a great deal of carbon deposits in the tailpipe. only happens when i tow. i keep it in tow/haul ,i know i'm not exceeding my weight. is it the oil i'm using? i use havoline 5w30 and a pure one oil filter.the truck runs great plenty of power, but can always use more. gonna get a k&n fipk one of these days! can anyone tell me about the excess carbon that the truck is producing? any help is greatly appreciated. this forum rules. thanx, demeat.