Assuming you are asking, KC, for the engine air filter element and not one of the other air filters on the coach, I can only tell you what our '06 C9 Monterey is supposed to use according to my manual: Donaldson P537448. I had also written down in my CAT manual "Model Ecolite 11X24 BI", which I may have gotten off the canister. The Donaldson number is likely what you want.
But you might be best off by calling Beaver Coach Sales in Bend and asking the fellows in parts, because your '05 may be different.
CAT's manual recommends new components annually, but I doubt most obey that. If the filter's flow indicator (aka Service Indicator, aka Air Filter Minder) doesn't show its yellow float approaching the red zone, I wouldn't get too anxious about replacing the element yet. If you do replace the element, be sure to press the reset button atop the indicator.
-Joel