I didn't do mine until I estimated I had maybe 10k on my 06 TT. Service guy said they were still in good shape but for $50 it was worth it before we set out for a long roadtrip. Not sure what they recommend but I check my boat trailer wheels every couple of years because they get submerged when you launch and retrailer the boat. Water will kill bearings.
I usually go 15 to 20k miles. Around 4 to 5 years. That is a good time for the brakes to be cleaned, adjusted. I ask myself the question " how often do the bearings on the car get repacked"?
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2011 Eagle 330RLTS with just about every option.
2017 Silverado 1500 4x4 5.3 with tow package. (no, we don't tow the Jayco with it.)
2018 Surveyor 265RLDS well equipped.
Life in the slow lane is still life.
While its unlikely, but if axles spend any time in water [flood, high water, towing thru stream] I would check them before long term storage or your next pull. Just pop the dust cover and check for moisture. Otherwise 2 years is a good schedule. Its not expensive. I use to do my own but as I got older I realized the cost is minimal and peace of mind knowing they were done right is worth it.