My Dad taught me to reverse a trailer. He wouldn't let me tow anything by myself until I could confidently reverse a trailer around the block.
The best bits of advice he gave that echo in my head (which might help your wife) when reversing are -
1. Turn and follow, turn and follow (ie turn the camper a bit, then follow the line with the car, then turn more)
2. You cant correct a mistake (or jack-knife) from an angle, you need to straighten up first
3. The slower the better
4. (and as mentioned above) Turn the steering wheel towards the mirror where you can see the camper
and for you:
1. Patience
2. Calm voice
3. Deep breaths
My suggestion is to go to a new land subdivision (if there is one near you - otherwise a big empty carpark) where there are kerbs, roads and footpaths, but no houses, people or cars. Let your wife watch (from outside the car), how the trailer moves when you turn it. Understanding the "arc" is important.
Dont ever ever ever yell, criticise, pressure, shout 'no' or 'stop'. It adds to the stress! When a reversing attempt starts to go pear-shaped, just ask her to slowly drive forward and straighten up and start again. If your camper is difficult to see behind the car, you could consider hiring an enclosed, small-ish, furniture trailer for a few hours. These are easy to see and so she can watch in the mirrors or over her shoulder how it reacts while reversing.
Remember, turn and follow...
I can reverse park our camper better than my husband
- and he knows it
- so I am always the one that does the reversing of camper into campsite or boat down the boat ramp