towing using lpg

Caravanman2003

has anybody out there towed with a range rover vogue 4.4ltr jag engine with 6 speed auto box(2006) using  lpg. I am interested in fuel consumption figures, as i am thinking of buying one,and whilst i am not expecting anything startling i would love to hear
anyones experiences. The figures i would expect solo are 17/19mpg petrol,and16/18mpglpg.What i want to know is what happens to mpg when towing on lpg