05 freightliner columbia om460 mercedes

This is the first truck I have ever drove with the Mercedes engine it is an old 2005 averirett freightliner Columbia it has a 10 speed transmission and of course the Mercedes om460 engine and I think it gets good fuel mileage don't have to get fuel very often but that's about only good thing about it it says on the Id plate on the motor 450hp or 460hp not for sure but it one of the 2 for sure and regardless of wich one it is its not reaching the max peak hp I think the reader ends are gear a little high to wich don't help the situation much with a 10 speed transmission because u have to almost wind 9th gear completely out to 2100-2200 rpm before u shift to 10th or it will just fall completely on its face iv checked all duel filters have checked all the air to air and turbo hoses have replaced injector pumps and still it will do decent every once in a blue moon but for the most part it don't matter if I'm pulling a empty trailer or a loaded trailer it's almost like the heavier the load it might let it pull little harder and put out a few more horses to try to keep it moving and still not pull worth a shut have to drop from 10th to 5th on 41000lbs no mountains just few minor pulls but then could go hook it to a empty trailer and u would think it would sling that trailer all over parking lot but it's almost like with the lighter loads it holds back some power instead of putting out full power or at least as much as did with a loaded trailer it cuts the power back depending upon the load itself Idk it's aggravating as hell and even when loaded and it's putting out power I don't think it is putting out no were near 450 does anyone know what my problem might be or know a few tricks I can maybe try to get few more horses out of it its sick as hell