The problem our trailers face are two fold.
First, they tend to suffer from lack of use. They sit outside, and temperatures change. This sometimes results in moisture condensing on the camper. If the bearings are not perfectly sealed, there could be moisture condensing in the wheel bearings also. The easist way to deal with this is to repack the wheel bearings and replace the seals every year or two on your schedule, not on the side of the road when they fail - or worse, having to have the camper towed into a garage on a flatbed to have the old bearings cut off because they were run without grease and they have fused themselves to the axle
The second issue out trailers deal with is neglect. I know, many here take fastidious care of their campers, and I try to as well. The truth is, at least for me, my trailer will sit in the back yard for weeks and I will hook up on Friday evening and hit the road without checking everything I should. I may have noticed a minor problem the last time I parked the trailer, and with the best intentions tell myself the problem will be fixed within a week. Then some other minor crisis happens. The trailer doesn't get fixed, and the problem is forgotten until it is noticed again during the next trip. Sooner or later this will bite you.
Bottom line: If you do nothing else to maintain your camper, monitor the wheel bearings closely. If almost anything else fails on your camper, you will still be able to hitch up and take it home. With failed wheel bearings you will likely get towed on a flatbed to the nearest garage.
As for the water heater; mine gets drained at minimum once a year, in the fall
, when the trailer gets winterized.
-- Dan Meyer