I was thinking and planning for some longer river trips, back when we lived in STL, MO, there are stretches of Mississippi when there are no services for hundreds of miles, I considered using a fuel bladder in the ski locker, where we can fit ~80% full 800 lbs ballast bag for wake surfing. I ended up not doing it once I figure out the 5gal jugs were just easier for an occasional use but a bladder would work very well with transfer pump, and not super expensive either.
As far as the fumes, for me the key thing that made it all work was to check (and fix or triage) the jugs for potential leaks prior to using them in the boat. That compartment has no electrics or electronics requiring ignition protection. And let's keep in mind what sits directly aft of the ski locker... the next compartment holding... a 50gal poly fuel tank. Incidentally, unlike the ski locker, the fuel tank compartment has a lot of electric, steering cables, and there is a potential for chafing etc. And yes, partially filled poly tanks are permeable to gasoline vapors in a small degree...
As far as the temperatures, the content of the ski locker tends to stay cool in my experience, probably because of water cooling effects on the bottom of the boat, I have never seen that compartment get hot. The locker hatch is shaded from the sun by the bimini, and can be open to check for fumes and visually inspect contents anytime. I used this setup in MO during summers when it gets pretty hot... and the big rivers can get pretty nasty, too, which brings me to the last point.
I would say the memory foam sandwich trick (large pieces of memory foam mattress from Sam or Costo placed over, under, and in front of the jugs) is important - it helps to keep the jugs from bouncing or moving; empties go back in the locker, and the jugs are just tight enough in there so there needs to be no room for the jugs to bounce around at all.
Those are my 2 cents.
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