X2 to Chuck.
RV tires/axles are under a MUCH higher load than your car or truck. Our Chevy weighs just over 5000# with fuel and 2 people. Our camper weighs 6858 empty as shipped. Same number of wheels taking up the load...
The stresses on an RV axle are much different and higher than those on your car (look at the difference in suspension, and you will see why - cars have much more complicated suspensions, which affect how the wheel's supporting member (ultimately the bearings) take this load). The higher load is also on the cheapest bearings they could get in bulk.
RV tires also have a MUCH higher PSI. Ours have 65-lbs. Truck has 45-lbs. When the bearings fail and anRVtire blows, it IS a bomb going off. I am a mechanical engineer, so I know some of this stuff...well...claim to anyway.
You may not need to do it every year, but certainly every so many miles/at least every 2-3 years. We are seasonal...so my bearings will not be touched for a couple of years. If you full time it or travel alot, you will want to do it every year.
If you buy a used trailer more than 1 year old, you should repack for piece of mind.