The shop may never get it correct.
The reason is "some" newer trailers are "very" cheaply made; including the brake system. :E
They use the smallest wire, magnets and shoes they can get away with. I've seen trailers you can't even get close to locking up at speed because of very cheap brake parts. Even at a full 13 volts it would barely slow the trailer down because of junk brake systems.
Do you have one of those trailers? I can't say, but I'm sure a competent shop can.
BTW, Terry's post was spot on. You should be able to stop your truck trailer combo almost a quickly as the truck solo. Good luck and let us know what the shop finds.