The power absorbed by rolling resistance will be proportional to velocity, but the force it produces IS (roughly speaking) constant as a function of speed. Similarly, the power consumed by by aero drag is proportional to the cube of the vehicle's velocity, but the retarding force is proportional to the square of the velocity.
To push a vehicle at low speeds where rolling resistance predominates, the force required is proportional to speed. More speed -> more force for the given distance --> more work/mile. Have you ever pushed a vehicle any distance? Tried to push it faster? I believe that you will find it takes more force to push it faster.
The required power to meet the rolling force needs is a constant * V^2. Aero force is a constant*A*Cd*density*V^2. Aero power is proportional to V^3. So total required road load horsepower is rolling constant * V^2 + Aero constant * V^3.
Work done/mi = power * Time. That divides the velocity out for a given mile getting back to work done = ~ (rolling resistance force + Aero force)* distance.
So one measures the coast down time. The vehicle mass (ignoring the energy in rotation of wheels/tires/brakes/driveline (you do do the coastdown in neutral? right?) times V^2 gives the energy at the starting speed and the ending speed. The difference gives you the energy expended. Divide by the time and you get average horsepower over that time. Multiply the required horsepower times the expected engine BSFC (at the appropriate engine speed and torque) * gal/lb (~1/6.15 for gasoline) * driveline efficiency gives you gallons/hr. Take mph and divide by the above and you should have an estimate of mi/gal.