Hi,
Firs of all right click on the tank, you want to use as RCS propellant. There writes "default" change it to "service module" make sure that the tank is pressurized.
Then right click on your rcs thruster select "Show GUI" and then choose your fuel type. It is already selected for Hydrazine so choose another fuel type then choose hydrazine.
Right click on that tank again you should see %100 Hydrazine click on that
