Some folks consider the sweet spot is the high gear RPM where they get the best mileage. Others feel it's where their vehicle feels the best.
For example My Toyota Avalon loves 75 MPH. It is real smooth, gets the best gas mileage and still has enough power to pass or hill climb.
My old truck towing the 5th wheel was 60 to 62 MPH. My new truck towing the 5th wheel feels best at 65 MPH but I get the best mileage at 55 MPH. So I'm usually at 60 to 62 in the new truck.
So the best method is to try to maintain a specific speed from one gas fill up to the next and then see what MPG you got. For the next leg of the trip add 5 MPH and measure the MPG again. Keep doing this until you hit the speed with the best MPG. Of course the road conditions and weather need to be similar for each leg to get a true picture.
Or you can take it to a place with a dyno big enough for your MH and they can tell you where your engine performs the best.