Stan, if the engine coolant in the expansion tank gets below the sensor, the sensor activates the check engine lamp, as a number of other things can do. But low coolant level is a common reason, so the first thing I'd do is check that your coolant is where it should be in the tank. A cracked and leaking hose below my tank caused a similar issue even when my coach was virtually new.
The tank on yours, if original, should be similar to mine - a white plastic one just behind the engine door, to the right of the engine and practically in front of your face, so it's easy to check. The tanks are known to crack, but be sure to also survey all the hoses involved with cooling. Many make long runs, especially from the tank to the radiator to the far left of the engine down low. Check the radiator for evidence of a leak also.
I can't say off the top of my head what the oil pressure should be, but more than likely it's fine and the issue is low coolant. The lamp going on and off intermittently is also indicative of coolant level low at the sensor, splashing around it on the road rather than fully covering the sensor as it should.
If tank or hose repairs are indicated, it would be a good opportunity to move to ELC (extended life coolant) if the coach isn't already using it.
Joel