I'm going against the grain here little and would like to suggest disc brakes for your trailer as an alternative to an engine brake.
Right now they are about the same price and performance is excellent. I'm in the middle of a 2 month cross country trip out west (our third such trip) and grades over 6% used to scare me to death now I almost look forward to them!
Stopping is never an issue, they operate more smoothly in traffic no wear on engine/transmiiion, no noise, one tenth the maintenance.
Of course the down side is if you tow other trailers or purchase a new one you lose the discs.
Give them a look.
Ron W.