I've struggled with this issue myself. According to Dexters info, the hubs are designed so the grease comes back towards you. If you watch while greasing the zerks, you can actually see grease seeping around the inside of the cup area if you understand what I mean. That being said, they do recommend you pull the hubs annually, though I've never done it. Be aware if you do pull the hubs, they also recommend you replace the seals.
2012 F350 Lariat Supercab DRW LWB Diesel
Reese Elite 18K - Trailair Triglide
2014 Sanibel 3500 - everything but the generator!