The electric brakes on the pop-up trailers need to be adjusted manually. On a car with drum brakes, the star wheel adjuster is automaticly adjusted when you backup and apply the brakes with a simple ratchet mechanism. The electric brakes on the popup don't have that. As the brake shoes wear, they no long grip the drum well. The star wheel needs to adjusted to get good grip. Another problem whit the electric brakes is poor grounding and damaged wiring preventing the brakes from getting full power. Lastly breaks that are very worn often have the electromagnet worn and it may need to be replaced.
Like I said, the Explorer brakes are OK, but not great. If you rely on the Explorer brakes to do all the work on on that much extra weight, you will wear them out, and seriously run the risk of the trailer pushing you around and fishtailing. You have a trailer weighing 3,500 pounds pushing a vehicle of only 4,400 to 4,800 pounds ( empty ). The trailers are designed to have brakes and in many states it is a legal requirement to working breaks on trailers of that weight. I don't know the laws in other states, but the State of Maryland requires brakes on trailers of 3,000lbs or more, with a break-away mechanism to stop the trailer if it becomes separated from the tow vehicle. About 10 years ago a young lady was killed in an accident near Annapolis MD when a trailer separated from a pickup truck and rolled in to the oncoming traffic and hit a small car coming the other way.
So do everyone a favor, and make sure your trailer brakes are working properly.