Check with your chassis maker for location of the oil pressure sending unit. Suspect it is on the passenger's side of the engine. But, oil pressure is not really something that you need to worry about with your engine. Oil pressure is used to inject the fuel. No/low oil pressure and the engine will not run-- kind of an automatic safety mechanism.
With that transmission downshift, use the shift pad to check the Allison for "Diagnostic Codes". And with the check engine light on, swing by a Caterpillar dealer to find out what happened.
An often overlooked item on your engine is the grease zirk on the fan hub (between engine head and fan). Access it only from above. Also, make sure that the water pump belt is properly tensioned. It is separate from the serpentine belt which is on an automatic tensioner.
And, as with any rear radiator, be sure to clean the front of the CAC as well as radiator at least once a year.
Other than that, just routine maintenance. Our last coach had 170,000 miles and was still in top condition when we sold it. But, it did get proper "care and feeding".