That is a loaded question a lot depends on tire size. Example a small tire like a 4.80X8 that used to be common on light boat trailers and pop ups were subject to tire and bearing failure all the time. The main reason was when you were towing it at 60 mph it was spinning at almost three times the speed of your tow vehicle tires or it was experiencing the same forces as your vehicle would at 150+ mph. The wheels on you camper are larger than that but they are still spinning 15 to 25 percent faster than your tow vehicle. This is why all good mechanics suggest repacking the bearings every year and checking your tire pressure before every trip. This is the difference between having an uneventful trip and being broke down on the side of the road in many cases. I used to repack the bearings on small tires I mentioned above 2 or 3 times a year just to be safe. In todays world I would not own a trailer of any type with smaller than 13inch wheels. I also use only EP (extra pressure) rated grease listed for wheel bearings.
I always thought it was right when you buy it before transport, and then again right before you sell it! (Unless it's been less than 90 days or 90 miles from purchase to sale!)
Seriously though, I wouldn't argue with wiartonwillie on the once a year gig for pulling the drum for inspection and some fresh lube. But to go the whole mile and go with total clean out, repack and new grease seals...I'm not to sure I would go that far if no parts were needed. Usually when you pull off the grease seal to clean the back inner bearing the grease seal get trashed. For me in the past, one beach trip and half a dozen local trips a year wasn't worth a total repack every year. But hey, I am usually VERY financially challenged so I can't spend the $50 every year it would take to do it right. Then again....Haven't ever had a vintage camper for more than a year either! LOL
Unless like Bill says, some of those super small wheels spin like crazy.
You'll be able to tell if the pack got water in them either by a rust color or white milky color of grease. Then a total clean and repack would be needed. You can also check behind the drum backing plate to see if your grease seals are leaking. If there is grease behind the backing plates facing the center of the axle, then you need to buy new seals and clean/repack whole job.
Good thing about doing the total job every year, you get to always inspect the bearings and can usually replace them if needed before complete bearing fail. (Not a pretty thing) So if ya got the bucks, I'm for yearly total clean and repack before first trip in early spring.
1963 Shasta Compact 1964 Dodge Dart 170, 1st year V-8, Push Button Auto 1993 Ford F-150 2011 Ford Escape
As Soup said "Good thing about doing the total job every year, you get to always inspect the bearings and can usually replace them if needed before complete bearing fail. (Not a pretty thing)"
Nothing is any more fun than sitting on the side of the road on a Sunday afternoon with a spindle damaged beyond repair 200 miles from home, knowing that had you checked your bearings you would be home now. What really makes this fun is your wife standing behind you in the hot sun telling you how happy she is as the kids scream. that will make the cost of a set of seals seem awful cheap.