First, the guy that "checked" your bearings actually checked nothing. You can't see the condition of all the metal parts thru grease.
Second, most manufacturers recommend 12,000 miles or 12 months for bearing service and brake inspection, you are way overdue.
Third, adding a small amount thru that worthless grease fitting will do nothing. In order to grease the outer bearing you would need to pump grease till the inner bearing is filled, then continue pumping till the cavity is filled and then old grease from the inner bearing is completely forced out thru the outer bearing. Just plain nonsense.
Personally, I would never put a new or used trailer is service till I completely serviced and adjusted the bearings and brakes. I have seen too much and believe you have been lucky.