All you need to do is line up one side of the coach. The other side will automatically take care of itself.
Decide which mirrors that you feel the most comfortable with and put a line on the floor using that mirror as the guide.
When it is time to back up just follow that line with the mirror and you will be fine. The worst thing you can try to do is chase mirrors and switch from one side to the other. As long as one side is where you want it, the other side will be also without having to actually monitor it.
Put some kind of block on the floor so that when your rear wheels contact it, you will know that you are as far back as you need to be. These can be attached to the floor permanently if you want.