I had always planned to add a second camera on the trailer and use the second input for the trailer camera. (Mine will actually take three; I had planned to use the third for inside the trailer to keep an eye on the horses. On the do list, but not there, yet.)
I agree. It is usually the first thing that I do. On one of my trailers, I added LED reverse lights on the back, so I could see where the back of the trailer was going. I have them set up to illuminate the far side, i.e. crossed, so when turning, at least one of the lights is lighting up where the trailer is heading. If I were doing it again, I might use an LED light bar. I have the lights switched so that they can be on for work lights at the back of the trailer, or on via a relay and the reverse signal.
All the best,
Peter