OP has received 10 years of service from his Nev-R-Lube bearings without maintenance. The original purchaser of his trailer paid close to $600 for the Nev-R-Lube option because it is a premium bearing. Had he paid for conventional bearing maintenance over the years the cost would have been $1,000-$1,500, instead he paid nothing. If he is uncomfortable with the lifespan he can buy new bearings off the internet $400-$600 per set of 4 and have a machine shop press them in and enjoy another ten years of trouble free service.
My Nev-R-Lubes run extremely cool. I do a walk around hand check every stop which I would also do with conventional bearings. They are just a little more than ambient which I attribute to brake friction from stopping the rig. I tow my Nev-R-Lubes with confidence rather than anxiety. Mine are also 10 years old and if I was going on a 10k trip I would probably replace the cartridges prior. I have 50mm bearings on a 5,200# axles. If I had 8,000# Nev-R-Lube axles that are most likely overloaded I would consider changing axles. The poster child for Nev-R-Lube disparagement is a YouTube video with an Elite Suites trailer obviously running too heavy for the axles. Owner blamed the bearings, should have blamed DRV for specifying the wrong axles.