At the risk of starting everything off again I've got some fuel consumption results whilst stationary.
Not for a motorhome but from my 2004 Passat 1.9 PD engine.
I used VCDS to report fuel consumption in L/he.
With a not hot nor cold engine (62⁰) it uses 0.8L just ticking over. If I turn on rear screen heater, all lights and a.c. to max (runs 100w cooling fan) that rises to 1L per hr. This additional load is 30% of alternator capacity - at tickover probably amounts to 30-40 A
Might be less consumption if engine was up to temperature but that wouldn't be the case if running engine just to charge
battery.
Presumably 2.8L Ducato engine would use more?
I hope this is useful to show that interpolation of mpg back to zero isn't necessary to get static consumption figures.
Maybe other have engine diagnostics to give accurate fuel consumption at tickover?