I think you must prove to yourself that your trailer brakes are working correctly. If they are lazy you're gonna work the truck brakes harder and harder.
You didn't state if new, or what type of brakes are on 5er, drum manual adjust, drum self adjust or rare disc brakes.
If used, start pulling wheels, check bearings, seals, brake hardware and shoes. Learn how they work and if self adjust, the adjustment is in FORWARD direction I bet.
Brake controller get a premium, not a price point unit.
How to evaluate brake effectiveness? My first choice after driving is to shoot the same area of each drum with IR temp gun. No clue what value is OK depends on how much braking you've done etc. But a cool drum after braking is not a brake.
Used 5er, brakes are shot? living in Rockies? scrap em and install hyd discs you'll be impressed.
Gary