The hubs are filled with oil to lubricate them, in lieu of grease. There is a port in your photo where you can check the oil level. Generally they take little to no maintenance other than verifying the proper oil level.
Depending on how much oil you are seeing you may have a slight leak, but I doubt it.
Check your Freightliner manual (if it's a FL chassis), or the chassis manufacturer's manual. There should be some reference on the oil-filled front hubs and what maintenance they may require.
'07 Winnebago Journey 34H - CAT C7, Koni's, MCU's, SS Bell Crank, Safe-T-Plus
'07 HHR Toad, SMI AFO, Blue OX