2008 Other Peugeot Models Oil Problem

I purchased a Motorhome on the 10 August. The garage did a full service before I brought it. I drove it home (50 miles). Went to drive off next day on holiday and screen read "Oil pressure sensor failure tunr off engine". I phoned Motorhome people and got told to take it immediately to a peugeot garage. I did this and they told me I needed a software download, 2 hours later we were finally on our way and we drove 150 miles to our destination with no problems. Next morning when starting the engine same message appeared, we went to the nearest peugeot dealership and were told that the engine had been overfilled with oil and had blown the sensor. They emptied the oil to correct level and renewed the sensor. Next day we drove another 100 miles when the engine suddenly died. I turned the key and the fuel indicator went up and down like mad, the engine fired up, I managed to turn off the road by which time engine died completely. We got relayed back home (4 hours). Peugeot took 2 weeks to look at it and have said that it needed a new ECU due to water getting into the engine a few months ago. They assure me that the ECU is not at all connected to the oil overfilling problem and they also assure me that having overfilled the oil and then me driving it would not have done any damage to the vehicle.
This garage and the place I brought my MHome from I think are in contact with each other so I fear they have been told what answer to give me!
What I want to know is what possible damage could have been done to the engine with overfilling it with oil and then driving it? Would some of the problems take months to appear? Seems strange to me that the ECU should suddenly go just after the oil problem. Please help me as I am really struggling here to get a helpful truthfull answer.