It sounds like the trailer's brakes are not providing enough braking power. Is this a brand new trailer?
Test the trailer by pulling the break-away switch, this should put 12 volts on the trailer's braking system and should offer a great deal of resistance to rolling.
I have a 15K Montana, I can not lock up the wheels (on paved roadway) with the break-away active but the resistance to roll is significant. This is normal for electric drum brakes from what I have read and been told.
The truck and trailer should work together proportionally stopping, the trailer should stop its weight as should the truck. Neither of the two should provide braking for the other.