You probably have Lippert's 6,000 pound axles featuring the Sure Lube hub system. This allows you to take the dust cap off, pump in grease via the zerk fitting in center of hub. Old grease will come out the front of hub. Turn wheel while you are pumping in new grease. Wipe off old dirty grease and replace the dust seal. You have to rotate wheel. That means jacking up the wheel off the ground to do so. There are lots of videos on youtube that show how to do this. Sure Lube, EZ Lube, etc. They are all the same in the procedure.
Problem with the system is once the hub gets full and pressure in it from pumping in new grease till the old grease comes out front, the rear hub axle seals sometimes fail and leak. This gets grease all over the brake shoes and the brakes get poor performance and eventually fail. Just be aware of this and have your brakes tested, inspected, etc.
Surf around here, other forums, facebook, etc. There is lots of talk lately about failing inner grease seals on the axle hubs on Lippert's 6 and 7 thousand pound axles. These axles are on many brands and it is not just a Sabre thing.