Mine stays in cockpit
We used to keep it below under the seat hatch, until we realized the sloshing action forced gas fumes out the vent, and they stay in the cabin and bilge since they are heavier than air. If you sleep on board (or cook) its not good. I also had the problem of aging gas mixed with oil. The six gallon tank fits perfectly if jambed between the seats, but was more gas than I usually used.Now I use a small 3 gallon tank and a small plastic jerrycan beside it at the rear of the cockpit floor. If I need the extra gas, its not hard to add it, even underway; if I don't need it, I pour the unmixed gas into my car. Usually you don't sit that far back because of the sheets, but even when I do, my feet don't seem to need to be where the tanks are.Short of building an isolated, ventilated, accessible compartment, I don't know what else you can do. A permanent tank gets into serious alterations.