The original, so-called "trawler" concept was for a boat that by its very nature is economical to operate. Whether it has a displacent hull or a semi-planing hull, the boats were intended for relatively slow, fuel-minimizing cruising. A semi-planing hull like a Grand Banks, CHB, etc has some inerhent characteristics that some people view as desirable even at slow speeds like 8 knots or so. The fact these hulls can be driven much faster with the application of a lot more power is also a benefit to some buyers but it doesn't mean the boat has to be operated that way.
But displacement or semi-planing, the true diesel cruiser concept is a very economical boat--- be it a single-engine, displacement Willard or a twin engine semi-planing GB--- when compared to boats that are intended to be run at higher speeds like the big Bayliners, Sea Rays, Eastbays, etc.
So if it was me and I was considering the acquisiton of a boat to spend a lot of time cruising in, be it coastal or inland or a combination of the two, I would be more focused on things like accomodations, configuration, equipment, and what it would be like to operate and maneuver and relax and eat and sleep on board--- things that have a much greater affect on my interaction with the boat than fuel economy.
While the difference of a mile or fraction thereof per gallon can indeed add up over time, given the other expenses associated with cruising the fuel cost differences between the typical, economical diesel cruisers on the market isn't going to be all that signficant to my way of thinking.
I have no idea what our fuel burn is. I know it's "about" five gallons an hour as that's what GB36s with a pair of FL1120s tend to get. We carry 400 gallons of fuel in five tanks with sight gauges so we are well aware of how much we have on board at any given time.
So as long as we determine we have enough on board to do what we want to do, that's as far as our fuel thinking goes. We're not going to back off a few hundred rpm and run at 6 knots to save a few gallons because running at 6 knots in the currents around here can be the equivelent of staying home.
If the differences between 2.2567491 nmpg and 3.1849073 nmpg was that significant, we wouldn't have taken up boating in the first place.