No need to disconnect trailer while on the scales.
At my local CAT scale which is usually busy if I were to disconnect a trailer while on the scales I may have some trucker that is waiting for the scales ventilate my head.
A bumper pull trailer requires three separate weights.
Truck and trailer with everything on board as if ready to go RV'ing.
Driver always in the truck or always out of the truck and not on the scale pads when weight is logged.
Fist with WD system engaged. Truck front axle on pad #1 Truck rear axle on Pad #2 Trailer axles on pad #3.
Pull off scales so others can use it and disconnect the WD system.
Do the same with WD disconnected.
Pull off scales and find a safe place to drop the trailer.
Weight truck only with front axle on pad # 1 and rear axle on pad #2.
I have the DW help me make sure each axle is on its pad and not bridging two pads. She is off to the side so her weight does not enter into any of the weights.
With this you will have the needed information to determine if you have the system set up properly.