Are you sure it's plastic particles? Could it be minerals/sediment from water?
We used to not run a filter in our camper. Then we got some water that created all sorts of issues. All the faucet screens were almost clogged, and the toilet valve stopped working. I took it off, cleaned it out the best I could and it clogged again. I finally just drained everything, rinsed the water tank, drained and then started running fresh water through all the lines, especially the toilet line and tried to get the chunks out. I ended up replacing the toilet valve, started using the hole house filter and didn't have any more problems after that.