... so do we know how fast the RNA degrades in sewer conditions?
Slowly enough to not matter. Unless something is degrading it, RNA can last years.
I’m pretty sure ribonucleases are found everywhere (even in sewage) so if enough are present it would break down all RNA fast. RNases come from skin and dust, for example.
lol no, rna degrades if you so much as look at it. dna is quite a bit more stable.