I have a 2006 Monterery with a Cat C9 so I'm not certain if the hydraulic filter is in the same location as on your 2003 model. But on mine, the filter is located inside the engine compartment very low on the driver's side...as you face the opened engine compartment, it is on the left side about knee level. It is also very visible and accessible from under the coach just to the rear of the radiator.
Jim