Wheel Bearings

All the salesman where I worked had utility trailers to haul equipment. They had the same axles as most TT. We repacked them at about 25,000 miles and never had one go bad. The DOT makes you have a maintenance schedule and ours was every 3 months for the trailers, they only needed to be checked and they were always good.

^^^^^^Ditto...or a little longer.

Every couple of years

Repacked at the two year mark. Now it has been eight years and 10s of thousands of miles.
Mine are probably due to be done again. Axle suppliers say 12 months 12k miles.

Maybe you could get him to skip bearings once and put some wax on it?