Workhorse 8.1 liter poor fuel mileage

Lmcneil wrote:
I have a 2005 Fleetwood Southwind 37c with the 8100 Vortec V8 with an Allison 5 speed transmission with 28000 miles that gets very poor gas mileage, about 6 mpg towing my boat or pickup and 7 mpg empty. I tried the UltraPower chip which seemed to do almost nothing and then installed the complete Banks system (headers, muffler, exhaust, Ramair, and the Auto power tune. A little more power, but still the same fuel mileage. This is driving between 60-65 mph. The boat weights about 3700 lbs and the pickup about 5000 lbs. I read about guys getting 8 to 8.5 while towing and even better empty. The motorhome seems to have adequate power, but still will not run with the Ford 3v V10. There are no trouble codes on the engine or powertrain modules. I do notice that the inside of the exhaust pipe is jet black and rubs off on your finger. The Fords exhaust I looked at are clean and only a very little brown on your finger. I have not checked any other Chevy 8100's. The motor starts and runs perfectly and outside of the super noisy thermostatic fan and the poor fuel mileage the coach drives and runs very good. Any help would be appreciated. Thanks.....

What kind of terrain are we talking here and most usually bragging rights of 8 or more mpg are on the flats. I've gotten as much as 9 on flat land with no head wind, not towing, no generator, and at about 52 mph, which is my normal speed when driving a behemoth of sorts.
Does sound like your engine is running rich though, so that would account for some of it. Are ya sure that all these mods you speak of hasn't changed things with the computer and it's sensors?