I know this site and its members seem to be heavily biased towards sailboats, and to tell the truth, so am I. But as I live in a region pretty much bereft of really good sailboat-cruising waters, practicality causes me to consider power cruisers.
Today we can find a few displacement cruisers, such as the Stambaugh Redwing series, or maybe Bateau's Maia-24, but if you look at their specified outboard's fuel consumption in gph vs their hull speed, the yielded mpg is not so hot.
Looking at plans from, say, the 1950's or so, there were plans available for lots of small 18 - 20 foot cabin cruisers that were powered with small outboards. I keep coming back to the 18' Puffin by Sam Rabl as a perfect example. One of these would be just about perfect for use on the "Western Rivers" - The Ohio and the Mississippi and their tributaries. Economical to run, light to tow behind a small to medium car, and fast enough to yield decent mpg, yet not so fast that hitting a floating beer can will hole them.
I have in mind something of about 18' LOD, planing hull, with a cabin just big enough to have a V-berth, a tiny galley, and room for a cooler and a porta-potty. With some lateral thinking, maybe the porta-potty could go under one of the seats in the cockpit, under the shelter of a Bimini top and drop-curtains for privacy. I know you'd have to keep it light, but stitch-and-glue plywood would seem feasible, or even plywood on frame.
I think a good turn of speed would be necessary, as with the downturn in the economy and other factors (EPA?), fuel does not seem to be as readily-available as it was in past years where just about every floating dock along the river had a fuel pump. Fuel in portable outboard cans might be the best solution these days, as during a cruise it might be necessary to beach the boat, load the outboard can on a two-wheel cart, and hoof it to the local gas station.
Anybody know of such a small cruiser suitable for, say, a 20 to 30 hp outboard?