I have friend that built a very nice trailer to carry his track hoe. They used 1 ton rear ends and cut the gear box out of the centers and filled the void with a pipe. They wanted the brakes and oiled wheel bearings. The trailer has been around for years now and works great. What they did that I thought was very cool is they used plexi glass for the outer hub cover so they could do a quick walk around and if the axle tube was full it was ready to go. But I assume they could do this because the old cover was the floating axle? so when they pulled that stuff all they were left with was needing a simple cover on the hub.
It also sees like i saw semi trucks with covers like this on them or the trailers. A visual oil check would be amazing for a good pre flight.
Trailer and steer axle sometimes will have clear covers if they're an oil bath bearing. Through the cover is how you would fill those so if they aren't clear there should be a plug somewhere that you can remove to check the level.