Had the boat trailer serviced at the shop the other day and noticed that the oil bath bearings are getting to be pretty popular. I noticed a display at the RV show and see they are used on all types of trailers including 5ers, TTs, and boats.
The inner seal is a double lip on the spindle, and the outer cap is a see through plastic that shows the level of the oil. The oil just bathes the bearings like a gearcase.
Pretty cool
http://www.tiedown.com/pdf/a751.pdf
The only problem I can see is long term storage, especially with any moisture involved.