Tank is metal. Coming from an Aprilia with the plastic tank problems, this was a big winner for me.
You can see the owners manual with service schedule here:
https://www.manualslib.com/manual/1422392/Ktm-790-Duke-2018.html#manual
You can see the owners manual with service schedule here:
https://www.manualslib.com/manual/1422392/Ktm-790-Duke-2018.html#manual