I had a Mainship Pilot 30. The reservoir was mounted nearby right next to the thruster motor. It should not need to be topped off. If it is losing fluid, that is not normal and there is a leak somewhere. So in your Mainship I would assum it is under the forward berth???
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B