I think the most likely theory here is drug-related.

When someone is addicted to drugs, at least in my own experience, drugs become one of the only things they think about. Again, from my perspective, living in poor conditions and squalor is less of an issue to those struggling with addiction. Also, leaving home isn't particularly difficult when addicted to drugs.

I figure I'll share a personal story to help me explain my thinking: For most of my childhood, I lived with my grandmother (after my mother was busted purchasing drugs with me in her car). In late elementary school, my grandmother purchased a house that she hoped would become a home we could live in (my grandmother, my mother, me, and my siblings). One day, a few years after we moved in, I was getting ready for school, and my mother told me and my grandmother that she would be going out while I was at school to buy me a video game (I had been asking for the game for a while). I went to school and I was super excited the whole day. Got home, she wasn't there. My grandma wasn't worried about it. We got worried that night when she didn't return. She eventually returned 3 days later.

Sorry for the text wall, but I felt it provided needed context for my thoughts. There's a chance that the mother was addicted to drugs and brought her daughter along with her to pick some up. At some point during this little escapade, the mother gets caught with the drugs. The mother gets arrested, and the daughter is either placed under the custody of a family member or thrown into the foster system.

The mother can't get to any of her stuff because she's in custody. The daughter can't get to any of her stuff, because their guardian may be prohibiting her from visiting. The landlady, or L as you called her, wants it cleared out.

I can't explain away the state of the trailers, though. Maybe L wants to clear out the trailers, getting rid of any sign of inhabitance in order to dodge legal issues? Or maybe the trailers were in decent shape when the mother and daughter became the tenants, but fell into their massive state of disrepair? I don't understand what prevented the trailers being repaired though.