We had a very similar problem a few weeks back. Recovered from Norfolk to Adams Morey in Portsmouth who discovered that part of the wiring harness had chaffed through under the dash board, just above the pedals. This meant the gear selector was not communicating with the gear box. Took them several hours to trace the fault but quickly and easily repaired.
The bad news was that they did not consider the fault to be covered by the 5 year extended warranty because the fault arose from the repositioning of the wiring loom by Carthago during their construction process. I was happy to pay up and get back on the road.
I took the matter up with Chelston who sold us the motorhome. They referred it to Carthago and within a few days the cost of the repair was refunded to me via the dealer. The fact that Carthago paid up so promptly suggests to me they already knew about the problem.
I suggest you get the wiring harness checked out. Picture of problem area here. https://www.motorhomefun.co.uk/forum/threads/carthago-compactline-breakdown.263073/
The bad news was that they did not consider the fault to be covered by the 5 year extended warranty because the fault arose from the repositioning of the wiring loom by Carthago during their construction process. I was happy to pay up and get back on the road.
I took the matter up with Chelston who sold us the motorhome. They referred it to Carthago and within a few days the cost of the repair was refunded to me via the dealer. The fact that Carthago paid up so promptly suggests to me they already knew about the problem.
I suggest you get the wiring harness checked out. Picture of problem area here. https://www.motorhomefun.co.uk/forum/threads/carthago-compactline-breakdown.263073/