Also not a plumber, but if one sink doesn't drain while the other is blocked, vaccum pressures could stop flow, indicating the system is sealed somehow.

can't tell by the photos, but if you have new pipes there, and you have the leveled correctly to drain towards the septic, it must be a deeper issue.

Once upon a time, I lived in a manufactured home. I had a similar issue.

Turns out while there was no trailer on the pad, kids dropped cement chunks into the drain pipe.

Sounds like deep blockage to me.