Ok thanks for the info.. On the isx we have alot of troubles with the egr valves, even tho they are not codeing on your ecm they could still be slightly closed due to carbon build up. This will cause your engine not to get the proper amount of air into your intake thus causing your engine to work harder to keep up and also cause black smoke to occur occastionly and when your engine is bogging down like at 1500rpm its not getting the best fuel mileage or power it can.. And also the injectors over time on these engines cause the same thing.. On these engine the injector get weak at a earlier time due to the egr process. But i would take off the egr vavle first and ck for exessive carbon build up.. That is your most likely cause since it has just started.. Also your truck should get better fuel milage under a load becaue of the designes of your engine.
Thank you for the accept and let me know what happens.. If you need anything else just come back and see me...