Thanks again for the recent advice - we just finished the epic trip ...
In the end we did a total of 6300 km of very mixed conditions ... both tarmac and off road with a final fuel economy of 6.8l/100km so all very happy that economy seemed back to normal. The little Italian mistress didn't miss a beat or misbehave the entire time and we loved bombing past all the heavily laden Toyota Hilux's chewing gallons of diesel on all the dirt roads.
Image from the trip below ... note the cunning full size spare and extra fuel tank arrangement