About 18 months ago this chewing began and nothing has changed. We live a quiet life on the farm with 2 male friendly labs. Topsy is female and a house dog most of the time. She keeps her eye on me and is a loyal companion. The vets treated her for fleas although no fleas were found. It is a mystery. She is diligent in chewing night and day and can hardly relax. The hair is gone from her back side, Nothing we have applied or given her has helped. She is a wonderful, fit looking dog with a healthy appetite fed once a day in the am with Old Roy.
Please help, Thanks in advance.

You feed her Old Roy? I'm sorry, but that is garbage food and she is most likely having a reaction to that. It's really no different than feeding your child nothing than McDonald's. No nutritional value.

Change the food to a high quality chow and visit your vet for a proper diagnosis.

Definitely change the food.
Did your vet do a skin scrape to check for other parasites?
What have you tried so far?

I have a JRT and she too has started chewing herself, wherever she can reach.
I attribute it to old age (dry skin) + winter (dry skin) + boredom from not wanting to go out much. When I yell STOP she stops, for about an hour or so.
I also bathe her more, maybe every 5 weeks instead of 10 weeks, because she gets smelly sooner, and her age I think means slower new skin replacement. I brush her too, something she avoided when younger but likes now.
I give her a cooked egg about once a week and some olive oil on her kibble, and a few fresh veggies.