The red hub seal removes, and you can find a replacement at a truck parts place. I found one at a Flying J that had truck parts. It seems to be universal. It was <$5. It just pushes in and pulls out, held in by friction. Mine was not very tight, and appeared to be causing a minor leak. I replaced it, and the leak stopped. The new one fit much tighter than the one I replaced.
A friendly trucker saw my leak, and told me the fluid should be near the level of the inside lip when you pull the seal out. If your hub is like mine, and I assume it is, you can see the fluid level without removing the seal. He warned me that if a truck inspection saw a leak like that on a commercial truck, they would pull him off the road. If it leaks and runs out of fluid, it can overhead and cause a fire or wheel loss. (Supposedly neither one is desirable. Go figure.)
'04 Alpine Coach 36' MDDS