My 2002 Thomas 5 window with only 2 seats and lift weighed right around 9600 pounds.
Got between 9 and 10mpg at 55 and struggled to keep that speed.
Put a 4 inch straight pipe from turbo back and a Live Wire tuner on it and got up to 15mpg at same speed.
Good luck and enjoy.
2002 E450 Thomas/ U-JOINT 4X4, family hauler
1954 Crown 35 foot RV conversion, guest house
1945? City Bus (unknown manufacture), yard art
1940s 'Binder 7 window, man cave