First thing is that the weight that is on an automotive bearing is a lot less than a RV. Second thing is that with a vehicle, you are using it most of the time, and it is not sitting in one place. A RV is sitting in one place with the weight sitting on it, which allows the grease to get pushed out where the bearing sits with the weight on it.
Also, in the 60's the vehicle manufacturers recommended that you service the wheel bearings every 10 to 12,000 miles or yearly, whichever occurs first. This was before the late 60's early 70's when the service periods began to be extended. Advances in lubrication technology and now with sealed bearing systems have changed the service intervals.
There is now sealed bearing systems for trailer axles available that require no service. When failure occurs, you just change the whole assembly.
I hope this helps answer your questions.
03 Silverado cclb 3500 drw, D/A, Reese 22k hitch
07 Alfa SeeYa Gold 30RL,2 slides,power everything