Diesel Bombers - New to the diesel world! Need some info!

Hello my name is Jared just purchased a 2003 chevy 2500 duramax back in November just recently hit 200k love the truck all though it seems to lack power in some areas. We have a few trailers some bigger and a few smaller. Haven't towed a whole ton with this truck besides a single axle flat bed somewhere around a 16 footer nothing huge. It struggles getting up to 70 down the hwy... My dad owns a 2004 f350 that he pulls our 5th wheel and 28 foot cargo around like a dream. Even loaded down to 15000 pounds pulls it 75 all day long without struggle. So seeing this I figure something must be going on with the truck. Tried replacing fuel filter put k&n air filter in it which doesn't seem to make any difference between that or stock. Does have 18 inch XD rims with mud grapplers so I know that will make a slight difference but nothing this major. Guy just suggest cleaning or replacing MAF sensor so going to give that a try. One last thing when I bought the truck it came with a predator programmer. Weather it is set to stock or high performance truck runs the exact same with no difference. Possibly something there may be an issue? Been trying to figure this out for some time now and ready to find a solution to make this truck run like it should! Help needed any advice would be greatly appreciated!:tu: