Straight line backing

Steer the trailer with the tractor.

 

I also have a spot mirrior (At least one somewhere) aimed at the drives under the front corners of the trailer on both sides. If that trailer ever so slighly gets out of straight and begins to make a angle you instantly add a smidgen of wheel against it and it should settle back down.

The slope of the ground you are on is going to influence where your ### end is going to end up. You won't believe how much the tinest slopes make a trailer come out of being straight. (The trailer does not know any different, gravity tells it go this way and so it goes.)