I agree. And "iron genny" aside, most moderate-displacement 36 footers will cruise at 4-5 knots at 1,900 RPM with a 30 HP motor, and will use about 1.0-1.2 gallons/hour. That a DAY AND A HALF of continuous cruising or 150 nautical miles (all figures are conservative).
If you are doing daysails or coastal, you are unlikely to run out of fuel, and, of course, assuming this is a sailboat we are discussing, you can always sail.
It is simple to carry two 5 gallon diesel jugs lashed to the mast or the rail. This alone extends your range by 10 hours of running time.
I would suggest you not worry about an extra tank until you've got a couple of seasons with the new boat under your belt. I have trouble on my 33 footer using 11 gallons of gasoline for my Atomic 4 per season...because it's a sailboat.