Update-

Re-filled water tank and drained water into some buckets. There was an opaque goo floating around in the buckets in small amounts.  I think what I thought was plastic bag scraps from the day before was actually this goo that had dried on the blacktop.

Could the bleach used to sanitize the water tank have turned like this after being in the tank for 4 days ?  A friend of mine thinks it might be RV anti-freeze, but I am used to seeing that colored pink ?

Obviously a little concerned now about using water from the tank even if just for cooking or showering.

Posting a new picture of what this stuff looks like while still wet.  The dark flakes are just small debris that was in the bucket and attached itself to this goo.

Sloppy or poor cleanup after constructing the unit.  This should have been taken care of at the factory - obviously wasn't.  Am surprised you haven't had any issues with either the screens at any of the faucets or shower head plugged or the screen on the inlet side of the water pump with the same issue.  May be a good time to check those areas as well.

Years ago I did see a similar instance where an individual left the drain to the FW tank open during winter storage.  Unfortunately mice got in, made quite a mess, chewed up a lot of the plastic tank and the result was small plastic debris, similar to what you have shown.  Hopefully that isn't the case for you as this individual had to replace the tank.  They used it for drinking like many of us do and couldn't get rid of the 'mouse pee smell' from the tank.

2019 Model 189

Getting ready to take maiden voyage.  Filled water tank and added chlorine for a good disinfecting.  When I drained the water tank, out comes a bunch of like plastic bag pieces.  In many years of draining water tanks on 12 different RV's - I have never seen this.  How would plastic pieces get into the water tank ?  Going to try and post a picture of what I found.