kitchen sink

[Dam Worker]

Looks like you may have to run a snake down the drain pipe. On most RV's the piping to the tanks do not have very long runs. If you already took the trap off you should have good access to the drain line. Other wise they make some cameras that you could run down the pipe to see what kind of obstruction is blocking the flow of water.
Only other thing I can ask is if you have two grey water tanks and if they are both empty?

Tom Marty