Originally Posted by Paul S.
It is not saw dust. It is particles from your anode rod in your hot water heater. Been down this road tooooo many times.
I'm pretty sure my toilet is not fed from the water heater!
The OP doesn't say age of trailer. If it's new the "saw dust" might be plastic
shavings from when they hole sawed the fill hole in the fresh water tank. Or it might actually be saw dust from cutting faucet mounting holes or pass thru holes in walls while open plumbing lines are laying around.
Anyway it sounds like a happy ending for Frank!