For example, how to wire a boat trailer so it will tolerate salt water, or replacing hub bearings. There's no motor but this is closely related to what you find in a car.
A boat trailer is, essentially, an accessory to a vehicle. So as long as the question relates to the operation and maintenance as part of the vehicle — i.e. something you might reasonably expect an auto mechanic to work on (wiring, wheels, maintenance, like you said) — it sounds sufficiently on topic to me.
I would find any trailer to be on topic with this site. You have to deal with tires, brakes, bearings, axles, lights, wiring, and proper setup with a towing vehicle and such. There are lots of maintenance and repair questions that would fit right in with similar answers to car/truck parts just with trailer specific instructions or things to consider.
This has some relation to the RV question: Are RVs in the scope of this site?
Trailers in general should be on topic here if the question is relevant to the electrical or mechanical operation and or repair.
Unless of course you are going to be programing on the boat while it's on the trailer, if so it clearly belongs on Stack Overflow.