I want to charge in-house battery (Fiat Ducato) from the motorhome solar system. I presume I can use a diode to do this? Can you please advise and what size diode would be necessary. Thank you.
As Solar Mike says, you need to look at split-charge relay systems for this - these are specially designed to protect both batteries by only connecting them when the inputs are operating and the voltages are appropriate, so that you don't end up draining the vehicle battery when using the domestic one or vice-versa.
To use both the solar system and the engine's alternator to charge both batteries may require a more complex relay system - I think it's more usual to use the alternator with a split-charge system to charge both batteries, then have the solar system connected to just the leisure battery (usually through the charge controller). I'm sure there are combined systems available to do both however.