Me and my friend drove 2 ohio yesterday to buy a 02 cummins. Truck has 64,000 miles on it, super clean. We drove the duramax down there pulling a trailer to bring the truck home with. We take the cummins for a test drive and it goes good. We sign the papers and purchase the truck. so we decide, lets see what this truck can do. We hook it up to the trailer and back the duramax on the trailer. We take off and get on the interstate, and the truck will NOT do over 65 miles an hour with the pedal all the way to the woods..We turn around and take the truck back..Dealer has no idea what the problem is...Any ideas to what the problem was...turbo?? One prob we saw is that the truck had a 2.5 in exhaust pipe..Needless to say the truck is still there and we are left disgusted..  

More than likely it has 4:10 gears.Check the tag on the diff.
I had the same problem with my 02 sport and I changed the gearing to 3:54s. made a huge difference in freeway driving and MPG!! 4:10s are fine if your pulling a big trailer around a ranch on back roads, but suck when your trying to go freeway speeds unless your gonna put 35s or 36s on that truck change the gearing to 3:54s I cruse at 70 pulling now no problem.
I also get 19-24 mpg at 65mph when not pulling.
Nobody gets 14-18mpg when pulling a toyhauler or travel trailer, maybe when they are pulling a flatbed with a couple quads on it.
Put on your hip waders if somebody tries to tell you different.  

Was the truck an auto? The autos are rated at 235hp...don't forget we are talking about a 2002! So now take 50hp off for the drive train and you've got 185 rwhp... not exactly big numbers, but it doesn't take much to add a 100 plus hp.  

4.10 gears would have allowed the truck to pull the trailer much easier than the same truck with 3.55 gearing. My truck with 4.10's and the low-geared 6-speed manual will top out at 90 mph before it runs out of RPM's and is on the redline. Regardless of gearing that truck should have pulled that load without issue.

Now then, when you were driving with your foot on the wood, did the engine rpm's go up or did it seem dead on power?

If dead on power, the first thing that comes to mind is a fuel delivery problem. Could be something as simple as a plugged fuel filter, although those trucks are known for losing the lift pumps, which is the small fuel pump that pulls fuel from the tank and supplies it to the  VP-44 main injection pump. When the lift pump goes bad, the VP-44 will draw enough fuel from the tank to run the truck down the road, but will be generally underpowered when more fuel is needed.

The problem is, without a fuel pressure gauge, you really don't know when the lift-pump goes south. The VP-44 was never designed to supply it's own fuel, which it uses for lubrication, and will eventually burn itself up doing so. Replacing both pumps will usually cost between $1400 and $1800 plus labor.

Now if the RPM's were climbing, but the truck wasn't going any faster, I'd be looking at a slipping transmission/converter.  

I'd agree with Dieseldude. It was a good thing you took it back because that truck had some issues.  Did you check for trouble codes?  

i think the wastegate was opening to soon it was fine without a load on test driving but when the truck had to actually do something the old wastegate would not take the pressure have it checked to see it would be a easier fix bd has one for like 185.00 not to bad i would bet that is what the problem is mine also has the 4.10 gears and it will top out at 110 mph in the red it usually runs 65 mph at 2000 or 2100 rpms with the 285 tires on it nomal mileage is about 17.5 with the 3.54 gears your will get better mileage without towing but really crappy when you hook it to something let me know if this helps