We have over 40000 miles on our trailer in almost 3 1/2 years. I check the bearings for play at most campsite stops and check temps. I repacked 1 wheel to check the grease a year and a half ago and it looked like new grease. It’s a triple axle and a little over 28000 lbs. I might repack them in the fall just to do it and the site will make it easy to do it. I definitely do not do it every year just because the service people say to. How often did you repack the bearings in the old rear wheel drive cars? Usually when the brake pads wore out.
However trailer bearing should be inspected annually for defects. Well, unless you're the kind that waits for a bearing failure first. And todays wheel bearing grease will settle out over time. I've seen it with one of my farm wagons that sat in a shed for 3 years. I removed the dust caps to see if they had any grease showing and oil ran out of the dust cap. Regardless of advertising all #2 non synthetic grease must meet the same specs.