This is a technical question from a very non-technical person. The new motorhome is based on a Fiat Ducato diesel engine, and it has only two positions on the ignition. Either on, (in which case the radio works) or fully on, for the engine to start. Theoretically, I suppose there are three positions because it can also be fully off.
In a car petrol engine, there are four positions: off, on (radio not working), electrical on (radio working) and fully on (engine fires and car goes). My understanding is that a petrol engine has a solenoid which can be burnt out – hence position 3 for the working radio and all other electrics. Presumably, this does not apply for a diesel engine – otherwise the radio cannot be used for more than very limited periods.
So this is a cry for clarification from a confused and technically challenged creature who likes listening to music.