All that's needed is for you to call ANY Freightliner Service Center and give them the last (6) of your VIN. They will tell you exactly what hub caps came with your coach and, they'll most likely have it for you the next day, if they don't have it in stock. They're not cheap though. I too lost the curb side front. I think I paid somewhere in the neighborhood of $30-$35 for it, maybe more, it was a while ago.
Now, as for WHY it decided to leave permanent residence on your coach, it's most likely because the spring loaded clips on the it were not taking a good purchase on the rim itself. If and when you get a new one, take a good look at those little clips. You can actually take a small screw driver (pocket style) and "tweak" those clips just a tiny bit and make them have a better "bite" on the inside diameter of the hole they sit in, on the rim.
While you're at it, you should remove the drivers side one and check the clips on that one. Note how easy or hard it is to remove that one. The harder it comes off, the better purchase the clips have on the alloy wheels. If it was somewhat hard to remove, then you might not need any tweaking of the clips on that one. If it came off real easy, then you're lucky because it too, could have taken a permanent vacation from your rig. So, some tweaking of the clips may need to be applied to that one. Hope this info helps some.
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '18 Honda Africa Twin Adventure Sports DCT
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Heidi character, (mini Schnauzer)!