I have not been able to alter my fuel economy by adding/subtracting weight, height, partydeck, or hauling a car.
There is almost a 4 thousand pound difference between the jacuzzi being full and empty and makes no measurable difference in fuel economy. I drove 250 miles on the x-pressway from my home in michigan to dayton ohio with the jacuzzi empty. I drove home with it full, and had the exact same fuel economy both ways.
I drove all the way from michigan-reno-LA-Michigan with my brand new roof rack with 8 empty 55 gallon drums on the roof, pulling my car, with about 50 or 100 gallons of water inthe jacuzzi, 4 people and all our stuff, and a 17 cubic foot fully loaded chest freezer among other things. I averaged better fuel economy on that trip than i usually get back home. I've found that here in michigan, i get 8.5 mpg on surface streets or x-pressway. On the big trip i averaged just above 9mpg.
The above is refering only to diesel fuel. The numbers are all slightly less when i run veggie oil
Other people may have a different perspective to share....
Quis custodiet ipsos custodes (who will watch the watchmen?)