Our 2016 ISL has the Fuel Filter mounted almost directly next to the fuel/water separator. Accessible behind tag axle, on the engine door side of coach.
Cummins Quickserve is a great resource. Freightliner also allows owners to register for DTNA online, which is a great way to find the VIN, Serial Numbers for major items, Engine, Transmission. Chassis, Axles. Also gives you access to chassis wiring, Air System, Parts lists. Difficult to navigate sometimes...esp since they keep changing things from time to time.
You black bag that comes with the coach may have the chassis manuals. They describe all of the service items and schedules. If you don't find them, they can be dow loaded from. FCCC (Freightliner Custom Chassis).
Any Freightliner Service and Parts location can lookup your Parts using the last 6 digits of the VIN.
Since you were actually asking about a service item...sounds like you must have gotten a really lazy Freightliner person. Freightliner 24/7 is usually pretty helpful. I could understand if you were asking about a fuel injector...but service Parts? They should know that stuff.
Not all Freigjtliner Shops are "Oasis"...so while they can still look stuff up, they normally don't stock RV parts...