I agree with the cop

Trailer needs to swing wide to make right turn. You assumed
it would make left turn. Truck driver does the wide right
turn all the time so it is reasonable to expect the driver
signal for the right turn. You made another mistake by
continue believing the truck driver would make left turn
therefore didn't step on the brake hard to make a stop.
Once the truck driver starts the right turn, you are in
his blind spot so he could not see you at all.