I instructed the tenant to take down all the curtains and wash in hot water with vinegar.

Then wash any bad spots, do one quick coat on entire walls that need it to avoid splotching. 

Not only is the house constantly filthy, the trash builds up and rarely is the house ever given a good cleaning besides dishes being washed, we aren’t treated with respect.

Might as well put in the document that the squatter must mow the grass and wash your car every week too.

Wash-rinse-repeat.

Or would this end up just being a wash against our primary residence?

You're not doing yourself any favors having that stained carpet in the house, I'm surprised your realtor didn't recommend replacing it off the bat.If that fence wasn't painted, you can have it pressure washed. 

They once offered up the basement for furniture storage for a charity drive... and also used the washing machine to wash about 20 loads of clothing they were going to donate. 

I say the same with garage door openers, wash machine too.

When she had company, she tore the paper plates in half and the napkins in half and washed the plastic forks for reuse.