Wheel bearings, and usually on a trailer that sits between trips. The grease separates, the seal lets water in, and the first sign is a hub that is hot to the touch at a fuel stop. Repack or replace them on a schedule rather than on a hunch. A bearing that lets go takes the spindle with it, and a spindle is an axle job, not a bearing job.

Because they age out. Trailer tires carry heavy loads at low mileage, and sunlight and standing still break down the rubber and the belts long before the tread runs down. Read the date code on the sidewall and treat it as the clock. Check the pressure cold before every trip, because an underinflated trailer tire heats up fast and comes apart at highway speed.

Nine times out of ten it is a ground or a plug, not a bulb. Trailer wiring runs through wet, salted, vibrating places, the ground screw corrodes into the frame, and the pins in the connector spread. A shop with a test box finds it in minutes. If you are chasing it yourself, clean the ground first, then the plug, then start looking at the harness where it crosses the tongue.

Yes, and they get skipped more than any other system. Electric drum brakes wear shoes and magnets, and the drums score like any other. Surge brakes have an actuator that seizes. Either way the breakaway switch and its battery need testing, because it only matters on the day the trailer comes off the ball. Have the whole system checked when the bearings are done.

Trailers in commercial service are inspected annually and the record has to be kept, which is why fleet shops run inspection lanes and sticker work as a routine. A pre-trip walkaround is on the driver every day regardless. For private trailers there is usually no scheduled test, so the maintenance you skip is nobody's problem until it is yours on the shoulder.