If you can keep the birds out of their coop for a few hours, I agree with ehs284, get the fumer. We had a REALLY bad infestation last year in our big henhouse. It is 32 foot long, and as I refused to look after the chickens at this time (long story), OH hadn't noticed the mites. He complained the hens weren't laying well, so I went in there, and Oh My, there were zillions of the mites. They were falling on my head, all over my trousers and blouse!!
So in desperation, I bought some of the biggest fumers, shut the hens out, closed the doors and popholes and cut open feed bags to cover the vents up. Took ages on such a big house. Then I set off the fumers. Left them a couple of hours then went back to inspect. Most of the mites were at the back of the house as that is where the perches were, the house is made of shiplap at the back, and the dead and dying mites were falling out of the joints in clumps! I've never seen anything like it. I still have one left for the smaller house we are using at the moment, just in case.
Then I swept everything out, poultryshielded again, and made the DE slurry u and painted ever crack and joint I could find.
All looks good at the moment, although we are only using one end of that house for 5 shaver blacks for now.
Shame there is no electric, I also bought a little steam cleaner. It isn't big enough for the large house, but I did steam clean the removable perches from another coop with it, and the boiling steam killed the motes on the perches.
Munty told me once to fill all the cracks and joints with vaseline, which I also did a few times. Any mites inside the cracks suffocate as they can't get out, and the others can't get in.
For the ground outside, what about one of those weed wands, like a flame thrower, but the modern ones use the little gas canisters??