Dexter recommends (pg 84) inspection and re-packing every 12 mo or 12K mi. Inspection includes removal of old grease with a "suitable solvent", so naturally re-packing goes along with it. Another reason to do it is to inspect your brakes and drums. I found an adjuster spring broken loose and floating freely inside the drum. There was some grooving from it. I called Dexter and they shipped me a whole new brake and hub assembly, no charge. I was well beyond my warranty. I had the old one turned and carry it and a couple extra bearing sets, in the aft toolbox in the garage.
If you haven't read the Dexter O&M, it is very informative and gives step by step instructions for about everything you need to know concerning their brakes, axles, and bearings.
Scotty and Kristen, Airedales Dagny and Wyatt
2007 Newmar Mountain Aire 4528, 450 HP ISM, Allison 4000, 8 Trojan T-105's
2014 F150 4x4 Crew Cab Platinum 157" WB
Roadmaster Blackhawk 2, RVI2 Brake System