Ted, bearing buddy's have been around for 50+ years and yes, are good on boat trailers due to the fact that the trailer wheels are immersed into water when launching the boat. The wheels are often hot from trailering to the launch, and when the hot wheel is immersed into the lake/river water, the drop in temp can suck water into the hub. The idea of the buddy is that the cavity being full of water proof grease (also used on boat trailers) leaves no room for much water. It works to a point, but even small amounts of water can cause rust that leads to bearing failure. Having said all that, it's not necessary on RV trailer wheels as they don't get dunked. Bearings also run cooler if the cavity is not packed full. The bearing rollers need to be able to roll freely and if the cavity is full, the rollers can't purge the excess and heat builds. Good for boat wheels, but not necessary otherwise. Save your money.
