PA-34 pilot reported engine indication problems during cruise. The pilot requested priority handling and diverted to make a precautionary landing.

Narrative

On the last leg of a 4-leg cross country; which started in ZZZ1; went to ZZZ2; and then was returning to ZZZ1 a few days later; I found myself on the ZZZ3-ZZZ4 leg; the last of the 4. I took off around sunset and my family was on-board my own personal Piper Seneca II. The airplane performance had been normal up to this point on the trip; and during this incident flight; was also normal through the takeoff and climb phases. After setting cruise power; the next time that I looked at the manifold pressure gauge; I noticed that there was a 3-inch split between the right and left gauge indications; but there shouldn't have been: all power and prop controls were even; and I know the plane well enough to know that that is not normal. I have a JPI engine monitor on-board; and I spent the next many minutes going through all of the pages; seeing if I could detect whether the gauge was correct and I was operating on something of a reduced power; or whether the gauge needles had gotten 'stuck;' or maybe frozen; and the engine was operating normally. Then; I discovered that the manifold pressure needle on the right engine; the one that was low; was not moving in response to throttle inputs.Though; the airplane seemed to yaw a bit when I played with the power; so I assumed that I had normal engine control; but the instrumentation was in error. I continued in this state for a few minutes; until I was directly overhead the ZZZ5; when I saw the right engine RPM gauge immediately drop to zero. I didn't feel any change in performance; so I figured this was also an instrumentation error; but I felt very uneasy because things are escalating: first the MP is low; then it is stuck; and then the RPM drops to zero. I was worried that the engine was coming apart; so I requested priority handling with approach control and landed without incident. After landing; a visual inspection inside the cowling did not show any obvious problems; or oil; or debris; or excessive heat signatures. The airplane will be examined in ZZZ5 in a few days when the mechanics can get to it; so I cannot give a conclusion as to what caused the incident. Thanks.