I'll throw this out and hope it helps , my DSDP with side rad, the P/S shares a pump with the hydraulic rad fan motor and the shares a resivoir and the ATF type 3.
Are you on a Freightliner chassis, Spartan ?? the chassis maker mounts the resivoirs so they would be the people to ask.
My resivoir is 10" in diameter and 15' tall. If you find the resivoir then the existing fluid color will be your guide , red=ATF, clear=PSF