Asda Gloucester - bought some yesterday!
It's impossible to predict just what consumption will be exactly on any particular engine - mine went up to 1L/hr with a heavy alternator load.
Whether you'd get the same load into leisure batteries would depend on cable size and for those who believe in them what sort of B2B you may have.
I don't think we should work on the basis of an idling engine being very warm - this discussion is based on charging on days with no sun and stationary vehicle.
I expect that larger motorhome engines (2.8L for many of us) will use more would it be 50%?
I don't know.
What I do know is that undersizing solar panels for regular use will need secondary charging and that carrying a separate generator and a container of petrol will eat into payload and storage space but using the vehicle engine doesn't.
I'd go for maximizing solar wattage and balance battery capacity against that.
Of course second and third batteries eat into payload and space - unless you've got lithium.
That should be good for another few pages of posts!!

