I had to replace one of my Vortex hub assembly's on my tandem trailer. the Vortex screw on cover cap had a hairline crack on it and was letter water seep into the hub causing the rear seal to go bad throwing off grease. I bought a whole new vortex asssembly and just installed it myself. There are two types of the vortex hubs- one is a straigt spindle setup the other fits a tapered spindle (wheel spendle is larger on the back and smaller on the front side. As far as I could see, there are only two units for the vortex hubs set up as described here. I went ahead a bought a seal kit, cleaned up the bad unit and regreased it, put back in the new package that the new hub came in to keep in my truck for a spare in case of an emergency.. The whole unit with everything it including the cover cap and seal- already greased was less than $80 new. To do this yourself- screw off the cover cap pull the nut keeper pin screw off the nut and pull the old unit off then just push the new set back onto the spindle and reverse the removal steps. Pretty much a no brainer- just don't overtighten the spindle nut that holds the unit in place- you can bind the bearings with too much pressure from the nut.
The Vortex hubs are grease filled- not oil filled. These are the only ones that I know about for Nitro/Tracker. When I bought my 2017, Z18, I got the Tandem trailer. Nitro might be using oil bath hubs on some trailers??
I've been happy with the Vortex system- I might have cracked the hub cover when rotating tires- I really don't know, but this is the only issue I've had and I"m at the end of 3 years running.