How to refill my EZ-Glide Hydraulic Tank

I know where my fluid tank for my 1997 RV"s slide is, but what is the proper level? I can't see any "fill" line, molded into the tank or printed etc. I've had the RV for 3 years and have never topped it up. I assume to fill with Dextron II like my levelers.
I have the user guide, but can't find the "How to Fill" data.

thx