Trying to make my mind KW, Pete or Volvo

I have 7 years experience and would like to know what the best truck is to buy today? I have a 2004 Freightliner Columbia Detroit settled with 14 l engine and 3.73 rear 10spd 650k, carry hazardous material average from 30 to 44k lbs and runs in most states of MA, AR, TN, OH. Flat and mountainous regions the truck makes an average of 6 to 6.5. Sometimes when I go to the west coast like UT, WA or OR it's 5.2 to 5.5 mpg at the pump. I'm unsure between Kenworth W900L, Pete 379 or Volvo 780. Cummins or CAT 13 or 18 spd. My question is, what is the advantage and disadvantages of each (MPG vs depreciation in 5 years of use) which will keep the price high. I think about buying a 2003 to 2007. MPG also relies heavily W900 and 379 they not very aerodynamic but they keep their price high even with high mileage. But I never drove any of them and do not have much information. I want to know in general what would be the best. I know the final decision would be mine but it's always good to have an opinion from the more experienced drivers. How much money do I need to upgrade as far as high flow muffler, monster stack, fuel wire, ECU upgrade etc..???

 

I realy like a Pete and KW they are classics, how can you make those trucks run at least 6.5 mpg? I can live with that. I really baby my truck, I don't hammer it down, I do progressive shifting and take care of it. I try to do all pre-maintenance possible.

 

I am trying to stay away from Freightshake. T-2000 is nice but also has a lot of electrical issues, my friend has one and he dislikes it.

 

How come you can haul that much? Van or Flat? Super Singles?