Sorry for the late reply ...been busy lately.
Yes, you can buy gas and ice, fill water tanks, etcetera at your destination if you need to although unloading those things from a boat that you'd like to take back down the right might present a challenge. Fuel weighs about 6 pounds per US gallon, so 100 US gallons weighs about 600 pounds. Water is more like 8 pounds per US gallon. It's better to have excess towing capacity than to have to little or too close to the limit. I know that my old 1/2 ton Toyota was rated highly enough to tow the same amount as a 1/2 Chevy pick up that I had, but on the road, there was a world of difference ...hated the towing capacity of the Toyota but the Chevy did great. The bottom line is that there is more to it than the numbers in the spec sheet. How heavy the tow rig is, how wide and long is it, how likely is it that the manufacturer 'barely met' or 'exceeded' the stated load limits etcetera. You've got to know your own rig. The same exact rules apply to trailers as well. Dual axle versus single, trailer width and tongue length, how much tongue weight you've got it set up to give you, etcetera. And boats don't tow like flatbeds with a load either, so there's no real way to test your rig ahead of time. The Great Alaskan is a lot lighter than other boats in its size category, but it's still a boat.
As far as sandwich construction for bulkheads go, I can't answer that without more information on the lay up, but if it is designed to meet or exceed the strength in the plywood that is specified, then there's no reason you can't use them.
Finally, yes, the primary strength necessary from the bulkheads is from the sheer downward. Building lighter from the sheer upwards should be fine as long as the bulkheads are kept reasonable. Again, it's hard to answer the question without more detailed information, but in general, your ideas are sound.
Brian
