Thanks for your input. I did all that and looks like the Volvo ground solenoid has been removed or was never fitted. The -ve from the battery is connected to a stud on the starter motor. The Onan does have a ground solenoid. Can you tell me what the reason for this solenoid is? Perhaps I should be refitting one to the Volvo circuit...but not before the engine is starting every time reliably.
Focus has now moved to the battery isolation switches. I found an odd voltage drop across the -ve switch of the 12v system. Have elected to bypass that switch for a few days and see if we get 100% starting. If so a new switch will be fitted. Will keep you posted.