That is a heavy 5th wheel........almost 12,000# DRY with 16,500# GVWR
Not surprised full lock up doesn't occur unless you are trying it on gravel/dirt surface.
Have you measured DC Voltage going to trailer brakes at the truck 7 pin connector and then at each trailer brake assembly?
Have you done a manual brake adjustment on trailer brakes?
Even new trailers need to be manually adjusted and then it can take several 'hard braking' events to burnish brake shoes/drums in for full braking effect.
Good information about electric brakes....
LINKBurnishing procedure for new brakes.......PG. 16
Setting brake controller up...............PG. 12
Brake adjustment..........................PG. 13
