Are the Maxxforce engines really this gutless?

My father bought an almost new 2012 International Workstar dump truck with 8,000 miles on it with the Maxxforce engine with Eaton 8LL- not sure if it's the DT or 9 model engine. He's had this truck for about a year and a half now and has always complained about the lack of power and terrible fuel economy. He even took it back to the dealer and after having it for 2 weeks they said they did some tweaking and gave it back with no change. I don't think its ever gotten better than 6 mpg- even empty!! I normally drive a '95 Freightliner Classic with 435 N-14 redtop cummins and I've recently had to drive his truck and I cannot believe how gutless this truck is!! I know it's a smaller engine than what I'm used to but this is just ridiculous!! This thing can barely pull itself empty let alone with a load! (weighs 26,500 empty) It has the twin turbo setup and seems like you NEVER hear or feel them kick in. Along with the engine issue, the a/c only comes out of the defrost now, and the "low fuel" buzzer comes on just because it wants to sometimes. The truck now has 18,000 miles and only getting worse.
We had issues with a previous '01 IH truck and after this one, I HATE International and am convinced they are total junk! My father is convinced its worth taking the hit and trading in this headache!
My questions are: are the Maxxforce engines this bad or could there really be something wrong? Is there any reason the turbos wouldn't be able to spool up? Why the terrible fuel economy?? Thanks!

 

I double checked this morning and its actually 2011 truck and its got the Maxxforce 11 in it! -370 hp and 1350 tq. I never would have guessed it was over 300 hp!
It's normally around 55,000 loaded

 

I know they were all up in the computer when the dealer had but don't know if they looked at anything turbo related. I would have to guess they updated the software but who knows. The problem is now the warranty has run out and my we'd have to pay this time. I would love to be able to put a boost gauge on it and see what the turbos put out- if any.