While I have one, the question is not quite that simple. The first Ford V-10 was 275hp. In 2000 that same engine was increased to 310hp with some changes in electronics and other things. Both of these versions used the same transmission, three speeds plus an over drive. Then with the new chassis just introduced this past spring, that same engin has been reworked to be 362hp and there is a new five speed transmission, with fifth now the overdrive.
With our experience, we have the 99 version, 275hp and four speed transmission. After one year we added a full Banks performance package which boosts the power by abuout 52hp and 62 ft/lb of torque. We are traveling full time in a 35' class A and we tow a Honda CRV. We normally travel at about 57-58mph and we generally average around 8.5 to 9mpg.
With any big block engine, the driver is as important to the fuel consumption as is the engine design.