Is there a calculator that can approximate the expected MPG

I use calculator...miles divided by gallons, each fillup. At end of trip total miles div. By gallons for trip average. Gas 2500, Avg 9.5, towing only..gas motorhome, Ford V-10 always seemed to be 8.0. Doesn't seem to matter uphill, downhill. Tail wind will give a tad better on a fillup, headwind wind a little less. Speed 55-60.