2003 Chevy C5500 rollback tow truck start and running smooth but not going over 55MPH

[Towexperts]

I have 2003 Chevy C5500 rollback tow truck only 122k miles on it was running smooth and go over 75mpg and up before start cold weather but since cold weather start it's start and running smooth but not going over 55 mpg speed.I change the oil and oil filter and fuel filter each time on 3000 miles.any one have Idea what need to do to fix this? some one said need turbo to replace some said need fuel injectors some one said just change fuel harness.I am totally confused any one can help me out please.