An earlier comment about me only having a 110 gal tank got me to thinking about where to put an aux 40 gallon (or so) tank.
The only place I can think of is to put a diamond plate tank on the transom extension. Something along the lines of 12 inches tall by 12 inches deep by 66 inches wide (40 gals, 248 lbs)
The motor is a Suzuki DF250 which weighs 580 lbs, the extension is rated for 950 lbs.
I've seen propane bottles on a transom extension (good idea) but never noticed a aux fuel tank.