The reservoir in my Monarch is up front in the center, behind the bumper. It is a cylinder with a filler cap on top. On the driver side end there is the pressure pump with lines going to the leveler control valve and out to the cylinders. The pump motor is visible in this photo, on the right. The motor resembles a starter motor. Yours should be fairly similar.
When the levelers are retracted, the fluid in the cylinders runs back into the reservoir. To lower the jacks the pump pushes fluid from the tank to the jacks. So, when filling, make sure the jacks are all retracted, to avoid overfilling. Most use ATF (automatic transmission fluid)
(Looking down from above)
