Adding power/torque to the 5.3 means either adding a supercharger or turbo. Not a cheap thing to do (like $5K for a complete setup). And then you may be setting your trans up for failure without a performance rebuild (another $1-2K). A tuner can only tweak things so far and in the long run you'd probably be breaking something. I have a duramax in my 2005 truck (crew cab, long bed, 4x4). It's one loooong beast. They're a very good engine and I pull my 31' camper with all my family's stuff in it and have no problems. I have a tuner on mine also. With it set on level one, once my truck hits about 48 mph and gets into high gear, the truck almost never shifts down. It just hunkers down and goes, even on fairly steep grades. I get about 12 mpg towing the camper which is about 11000 lbs. loaded down. Overall fuel economy is about 18 mpg combined not towing. Good luck.