Mark,
It sounds like you are dealing with 2 different problems. First the intermittent drop in pressure for the dash gauge should be involving the analog signal from the remote oil pressure sensor that is a different circuit from the digital circuit that the engine ECM is using to turn on the check engine lite. Unless in 2009, Monaco used a dash cluster that is tied into the Cat ECM system that I have never seen, a dash cluster can not cause the problem that you described.
There is a possibility that you are just looking at your dash cluster that is getting an analog signal instead of the digital signal from the Cat ECM that you would see on an Aladdin or a Silverleaf/Silverleaf PC. If so your "mechanic" did not diagnose your problem, he was just throwing parts at the problem. First you need to determine if you have two oil pressure sensors just like every Beaver coach that I have ever worked on. If so you need to trouble shoot the analog system that is feeding information to the dash gauge. You may very well have a dash cluster problem, but you may just need a wiring repair or a $20 sensor.
Gerald