Certainly, there is no practical way to enforce not littering. Are you going to have undercover cops patrolling around waiting for someone to drop something on the ground?
I live in a formerly-quiet neighborhood in Seattle. When my kids were 4 and 6, I had to teach them what a used needle looks like and to never ever touch one because trash like that started showing up on our street corner near parked RVs.
1. There are definitely some people living in vehicles that are not part of the drug trade as consumers and producers, and I'm very sympathetic to their plight, and supportive of them living in their vehicles if that's all they can afford.
2. There are definitely people living in vehicles that are drug addicts. I'm sympathetic to their plight, but it doesn't seem like letting them live without treatment is good for them or the surrounding community. This is a profoundly hard problem — how as a society do we deal with people whose addiction prevents them from making good life choices and upholding the social contract? How much agency should someone have when they use that agency to harm themselves and others?
3. There are definitely people living in vehicles that are drug producers and sellers. They should go to jail.
People who have strong opinions on this seem to believe only one of those categories exists, and then imagine solutions that deny the existence of the others. That's not practical. My experience from my street is that there are all three kinds, though unfortunately not many in the first group.
I care about the rights and life of people in these dire straits, but I also care about my own rights and those of my family. I'd like my children to be able to play in the yard without worrying about them rolling onto a dirty needle. I'd like my wife to be able to garden without finding trash and sewage in the bushes.
Even if someone is 100% supportive of letting people experiencing homelessness get by however they can, I hope they understand that it's not biologically safe for humans to live in areas without sewage and trash infrastructure. That's how you get cholera, hepatitis, etc.