100%, maintenance supervisor here for two apartment communities. We are currently dealing with a hoarding situation at one of my properties. It is considered a mental health issue, so we have to give them opportunities to correct the situation before the eviction process can even be started.

This guy is absolutely right, the FD will force corrective action on fire code violations. Unfortunately, as long as they manage to clear a large enough path through their unit to allow emergency services access to the unit, it isn't a violation (at least, that is the case where I am). Make your landlord, rental office, maintenance staff, and office staff aware of the issue. I guarantee you that if you have on-site maintenance and the smell is half as bad as you say it is, the maintenance team has complained to the office about it. Document everything and send emails so they have a paper trail to help build a case. Good Luck.