A couple of years ago, I was exactly where you are so I think I understand your question. I have the advantage of having towed sailboats for many years.
Let's consider lights from two angles: a legal one and a practical one.
Legally, I understand that anything you tow needs to have lights on it to warn drivers behind you of your intentions. This includes braking and turning. While you are correct than a toad brake system should turn on the toad's on brake lights, that doesn't address the turn signals. If you ever had problems with the toad's brake system, you would still need to have some sort of brake lights back there to indicate what is happening in the MH. This is normally accomplished by a 4 wire harness from the MH light system. Sometimes, those 4 wires are routed via diodes (to separate the MH and toad electrical systems) to the toad's regular lights. In my case, I route those wires to a set of LED magnetic lights on the trunk of the toad.
Practically speaking, I've never taken a trip in the MH without having someone tailgating me frequently. Whether it is getting close enough to pass me on a two lane road or just being stupid and following to close when they could pass in a 4 lane situation. Many times they are also not paying much attention while tailgating. I've had 6 times in the past two months where someone was directly behind me and I could move to the inner lane, slow down significantly and pull back into the right lane - without bothering any other traffic. That is quite a statement considering we are 55' long with the toad. For situations like that, I want all of the lights that I can get and thought that the LED lights offered advantages in brightness and getting the lights directly into the following driver's field of vision over the standard taillights on our Saturn. I particularly didn't like the contrast between the normal running lights and the turn signal/stop light filaments on cloudy days. I admit that I'm paranoid about someone rear ending me. I'd use airport beacon lights if I could find a way to make them fit on my toad!
I hope this helps.