If you can't find a large after-market type water filter, than you likely don't have one. They're hard to hide.
The screen filter shown on the left in Bill's kindly proffered photo is similar to what was on the pump in our old '84 Pace Arrow. It's merely to prevent debris from getting into the pump and fouling it, like the screen washer on a washing machine hose - it's not a carbon-type filter. It just unscrewed so I could clean the screen annually, just to keep algae and gunk from growing on it during storage, etc. But he's right... if that's an original pump, the screen filter housing may have dried and cracked from age or the seal is shot. Replacing the filter is cheaper than an entire pump, or you may be able to locate an O ring if that's all it needs.
But the pump itself on the Pace Arrow gave up after a dozen years, with internal corrosion causing leaks, so I replaced it with a better one. There are many now that are more capable and quiet, and it's not a difficult job to replace one.
-Joel