For a diesel pusher since you already have your tow bar M&G
If you had not already gotten your tow bar Ready Brake (With the matching tow bar)
For MH's w/o air brakes US-Gear unified brake decelerator
Those are my top suggestions,, Next is
All the other INSTALLED systems that are BOLTED into the vehicle.
And on my NON recommended list are things like Even Brake, Brake Buddy, and so on where you put a box in the driver's seat and it pushes the pedal.
Too easy to screw up if you are in a hurry or even to just say "Forget it" and regret later.