Last fall my girlfriend rented a room in a mobile home. Throughout the whole winter her kitchen sinks never drained very well. Her landlord got plumbers to come in halfway through the winter to try and fix it. They found a bunch of plastic and food and whatnot and after they left the sinks worked for about 2 days and then started clogging again. Due to the gurgling and gluging sounds it made when it tried to drain, I was pretty sure that the problem was a frozen vent. Since she was renting I didn't want to go under the trailer and open up the insulation to find where the vent was. Plus we often didn't cook huge meals here, so even though it was annoying we could still do the dishes once a day and the sink would usually drain by the next morning so it wasn't a huge pain. Recently though, her landlord got her boyfriend to come and open up the sink to see if they could fix it (neither of them are plumbers by the way) when they opened up the sink drains they found a bunch of plastic and food scraps clogging the drain. Even though we explained to the landlord that we use a plastic sink strainer every time we drain the sink and that we were very careful after the plumber came in last time, she is still blaming us for putting stuff down the sink drain. So I am wondering if anyone has any theories on how the plastic and other junk got there, because I am 100% certain that it wasn't from us. One idea I have and was wondering if it is at all plausible is. Since the vent was probably plugged, to me it would make sense that as the water tried to drain it would have to vent any air in the pipes back up through the sink. As the air was being forced back to the sink would it be possible to also slowly bring back up and crud that was already in the drains. Another idea that I am not sure about is, it has been about 10 degrees Celsius for a few days so the vents would have thawed, could plastic and stuff have gotten into the vent over the winter and as it thawed would it fall through the vent and get pushed back up towards the sink? If anyone has any ideas feel free to wright back. Thanks a lot.

I didn't want to go under the trailer and open up the insulation to find where the vent was. Good thing you didn't because the vent's located on the roof above the kitchen area.

Due to the gurgling and gluging sounds it made when it tried to drain, I was pretty sure that the problem was a frozen vent. Bang on! A gurgle indicates a blocked vent where the discharge from the sink
Vents though the sinks trap, Hence, the gurgle.

when they opened up the sink drains they found a bunch of plastic and food scraps clogging the drain. Where did they find this trash? The trap or the drain line?

after they left the sinks worked for about 2 days and then started clogging again. Sinks clog up if there's no vent to assist the drainage.

One idea I have and was wondering if it is at all plausible is. Since the vent was probably plugged, to me it would make sense that as the water tried to drain it would have to vent any air in the pipes back up through the sink. As the air was being forced back to the sink would it be possible to also slowly bring back up and crud that was already in the drains.
Good thinking! Shows you gave some thought to the problem. However, vents do not "push" air back up the pipes. They draw air in to relieve the vacuum set up by the draining water. In short there's no backpressure involved unless there's a blockage. Please describe the crud that was in the pipes and since the sink didn't drain all that well from the git-go how can the landlord know it wasn't there when she moved in?

Another idea that I am not sure about is, it has been about 10 degrees Celsius for a few days so the vents would have thawed, could plastic and stuff have gotten into the vent over the winter and as it thawed would it fall through the vent and get pushed back up towards the sink? I like this one. Some scoundrel snuck up on her roof when the vent was froze up and jammed "plastic , food and whatnot" in the vent so that when the thaw came the crud would fall down in her drain. One thing I'll give you, you're creative. But who knows how long that crap was in the drains?
Not you, not me and certainly not your landlord. Good luck, Tom