The main factors that determine speed when towing a caravan are:
1. The human brain of the driver and,
2. The pressure of the foot on the accelerator.
Speed and fuel consumption are definitely correlated as explain in this excellent article from Australian Business Auto.
My personal rule is not to exceed 90kph on the open highway and I find that not only is fuel consumption considerably lower than driving at 100kph but I also use my brakes much less.
The other reason I drive at this speed is because I am hoping to spare my grandchildren the trauma to dramatic climate change.
Driving at 90kph definitely reduces greenhouse gas emissions.