Hi, sorry to hear you have mites. They absolutely will kill your chickens, very slowly. I just had one of my hens die from what I think were mites.
You will need to check ALL your chickens for mites at night, since that is when they come out to feed. Get a good flashlight and a magnifying glass. Check all your hens for mites, part their feathers to get a good look at the skin, you will see them crawling around. Look around their vent areas, under wings, back of neck.
To treat for mites, you will need to do a COMPLETE cleanout of ALL bedding, roost bars, the entire coop (burn all bedding if you can), and spray every crack, corner, and crevice of the coop with diluted Permethrin-10 (I am not sure where you are located, but in the USA, you can buy it at Tractor Supply:
Gordon's Permethrin 10 Concentrate). You will also need to spray each and every bird thoroughly with the diluted solution as well, it has to be diluted according to package instructions for direct contact to the bird or else it can cause burns to their skin. You will need to treat the birds and coop once a week for 3-4 weeks to ensure all mites and larvae/eggs are killed. Diatomaceous Earth will NOT kill the mites, you need to you an insecticide like Permethrin-10.
Once you get your new coop, I would recommend treating it with the Permethrin as well before you add in your chickens and bedding.