HR Imperial 1998 differential oil change

[Norcenark]

My differential is a 4.63. Drained right over 3 gallons of oil. Going back with mobile 1 75/90 gear oil. Had 2 plugs on side of diff. One is a square plug and the other a few inches below is a 3/8 drive plug. I assumed the larger square plug is the fill hole. Also assumed the 3/8 drive plug was a fill to hole, but it only took about 1 1/2 gallon when it started to drain out the lower hole. Am I to fill it to the higher hole? Just want to make sure it wasn't overfill before. Thank Mark

[Spk64]

Not sure if the Roadmaster Chassis is same but on my Spartan need to make sure RV is level and air bags at ride height.
When air bags are down differential rolls forward and fill hole is considerably lower. I had this happen when changing fluid.

[jacwjames]

Attached is a lubrication manual for the Roadmaster chassis, it has a picture showing to fill it to the upper hole. Page 16