I have had this truck into several dealers and they haven't been able to fix it. It has no power, feels like transmision slips but the trans gauge doesn't overheat, the turbo whistles and the temperat

I have seen several questions that have had some of the issues I have been experiencing with my 2005 6.0 diesel F350 Lariat but not exactly what I see. My problem is that there doesn't seem to be very much throttle response from a dead start or acceleration while driving. When pressing the accelerator pedal to engage passing gear, sometimes it will engage and sometimes it won't. The motor just doesn't seem like it is very powerful. While pulling my 5th wheel uphill, it starts out fine but soon after into the climb, the RPMs will start racing, you can hear the turbo whistle, there is no increase in speed and you can hear the fans come on and it sounds like a 747 driving overhead. The temperature gauge rises to the overheat mark but never goes over it. It feels almost like the transmission is slipping since there is no increase in speed or feel of power. If I let off the throttle, the temperature gauge will immediately drop to normal operating temp and then the truck will pull fine until I get into the throttle again and then the whole sequence will start over. The transmission temp gauge never heats up. This happens sometimes with out my trailer attached but while making a climb. I never get and engine light and I am told that the dealer technicians never see an error code. So, the different dealers have played "Swap a part" at my expense and the problem continues to exist. To date these are the parts they have replaced. I am not "Diesel Mechanically inclined so I am sure I have been taken advantaged of. Replaced were New Turbo, electric fan sensor(s), fan clutch, turbo hoses and clamps, EGR Cooler, Intake Manifold Gasket, Radiator overflow tank, Oxidation Neutralization cleaner, Oil Cooler and Degas bottle. With all this changed out, the problem still exists. I bought this used with 48,703 miles on it and it currently has 70,500 miles on it. Any ideas?