Topping off battery distilled water every six months or so, checking air pressure in the tires every trip and replacing the tires at 4 years, the roof as mentioned. If you keep the trailer clean or under a cover, it will stay looking good however if you leave it without washing, black streaks going down the sides will take a lot of elbow grease to get off. Greasing the suspension pivots if applicable once a year. Sanitizing the fresh water system with a half cup of bleach if the trailer has been sitting (No need if the trailer is used every six months or so.
I myself don't advocate doing bearings every year. I think that is going far and above what is necessary and if the owner doesn't set the axle nut to the right tightness, can cause premature bearing wear.
