Welcome to the forum. Diesel fuel has a very distinctive smell so it should be easy to determine whether what you are seeing is diesel by dabbing a bit on your fingers and then smelling it. It has a very different odor from that of petrol.
Also, this forum is North America - based and Toyota has never sold diesel RAV4s here, so you might also wish to consult a U.K. = based forum such as:https://forum.rav4driversclub.com/ There are others as well.

 

Hi all
I have a leak under the back passenger side door left side on my Rav 4 2009 Diesel Sr180 Looks like diesel!!
Can anyone help me to identify the problem?
There is a box and a black hose leaking from the hose.
First time on a forum.
Thanks

Is that the fuel filler tube?
If so replace it

 

Really don't know!!!
but will check it out
Thanks

 

Parts diagram can be found/ seen:
Body/Interior | Toyota RAV4 ALA30R-AWFGRW ACA3#,ALA3#,GSA33,ZSA3# | Parts Catalogs | PartSouq
Most probably filler tube is corroded, part nr. "7720142150", but check with VIN first (on Genuine Parts Catalogs | PartSouq )

 

Yes I checked is that what is called the neck hose.
Thank you for all the help 😁