I suspect that the Sea Ranger you are looking at is the 75' in San Diego, right?
That boat has a listed weight of 55 tons and might easily be 60-65 tons with fuel, water and gear.
I agree with Fred's analysis with one exception: a semi displacement hull, which this seems to be, will use more fuel, at least 4 hp per ton. So it could easily need 250 hp to push it to displacement speed, which will certainly use 15 gph or better of fuel.
Displacement speed for that boat is about 11 kts. Slow down to 8-9 kts and you will burn half the fuel.
But to do better than that, get a smaller boat, not just smaller engines if you want good fuel consumption.