Some of those "J" shaped oil fill extensions extend/pull out.
I always removed them and installed a Caterpillar oil fill plug on the valve cover. Lots less chance of dirt getting into the engine.
Now, if it takes an hour to access the under-bed area/valve cover, I would probably rethink what I did.
BTW, be sure to CALIBRATE THE ENGINE OIL DIPSTICK. Caterpillar did not spec or install the dip stick, and it could be off.
Call the Caterpillar RV Hotline with your engine serial number to determine the correct quantity (both ADD and FULL) and calibrate the dip stick at the next oil change. Calibration is free and only takes a few more minutes than a standard oil change.
Caterpillar: 877 777-3126