I have a 1999 Ford F350 7.3 diesel with turbo. I has Ford service dept. scan the truck and it came up that #1 injector needed to be replaced. I replaced the injector and I had it rescanned and it came up with the code P0301. I tow a 30' living quarters horse trailer and get 10.5 to 11 miles per gallon. At a speed between 40-45 the engine runs a little rough. Can someone solve this problem for me

Reset the PCM with a code reader or scan tool and see if the code goes away. It's indicating a misfire cylinder #1, which is where you installed the new injector. That makes me think the PCM may not have been reset afterwards.