I'm not judging them for not having dental insurance, I'm just saying if you are from San Francisco you will have huge culture shock just driving 4 - 6 hours north.When I was in Crescent City, that was during the 2016 election and there were literally huge altars to Trump, too - giant wooden structures with huge decorated Trump signs/banners, etc. 

The only thing I can fathom is that in their own house they are on some kind of septic tank that doesn't take toilet paper, and they follow the same hygiene practice when they stay elsewhere.

Maybe make it a dental office?

You treat your dental practice like a business, you have to do the same with rental properties.There are also lots of good books on rental real estate, my favorite is "Real Estate Investments and how to Make Them" by Milt Tanzer.

With that said, I am not sure what can be done to combat the stink, if the tenants do not take care of their hygiene.

For the ones remaining, they will have to raise their game in terms of hygiene, cleanliness, potentially invest a ton more in reusable’s like linen or towels.

For me, I have access to a few hundred dental students I can rent to who are academically elite, responsible young adults.  

And we’re not even over 40 yet, so it will only get more expensive  I pay about $1500 for health insurance + dental for a family of four.

Housing prices shot up, and it the last 2 years, duplexes in our area went up $100K each year.This year we all went to the dentist, all 4 of us, and we need root canals, crowns, and even though with dental insurance, our co-pay is considerable.

This is what I'd consider spam.As long as you're scrubbing your numbers against the DNC, taking people off your lists when they tell you, and maintaining proper list hygiene, I do not see an issue with cold calling.