Hi and welcome!
The HiLo library of manuals is here: https://www.hilotrailerforum.com/f49/
If you can't find a manual for your 1989 model, this is one for a 1990 version: https://www.hilotrailerforum.com/f49...-classic-2016/
HiLo published "generic" manuals for each year's models so that one works for ALL 1990 models. Additionally, HiLo did not change their trailers much between model years in most cases, so I suspect this manual would work for you.
If you have a brake that's engaging by itself when you plug your tow truck into the trailer, that brake is receiving electrical power somehow. I believe you when you say your truck is not at fault - that would engage ALL the brakes. But, there is a single blue brake wire from the trailer plug that is delivering power to ALL the trailer brakes - it should split back near the axles, with a feed to each brake. Do you have braking to the remaining trailer brakes when you step on the truck's brake pedal. Possibly, they are inop and the only one working is the front one on the curb side. What happens to the brakes when you disconnect the tow vehicle and pull the safety breakaway switch? This should fully engage ALL brakes.
This can be a difficult thing to trace, but if the other three brakes work properly, simply disconnect the bad one from its blue input wire and start looking for what is delivering voltage to it with the TV connected.
I don't understand what you are asking about separating the bathroom. It's in its own little room, isn't it?