A:  One thing you don’t mention is whether the fuel is diesel or gasoline. One must be extremely cautious when contemplating drilling or cutting into an empty gasoline tank. If exposed to a spark, even a small amount of leftover gasoline fumes will turn the tank into a bomb! Get the advice and help of a professional when dealing with a gas tank.

As you’ve suggested, the tank can be cut out in pieces (subject to the above), or the cabin sole removed to allow removal of the tank in one piece. Both require the dismantling and rebuilding of permanent structures, which will be costly and messy.
The bladder tank option may be the least expensive, and the least disruptive. Bladder tanks are available from several manufacturers. This type of tank is used in the military, hazardous materials handling and in disaster response, as well as various commercial and pleasure applications on both land and sea.

If the top of the existing tank can be accessed, there should be room to cut and install a large access plate in the top of the tank (again, note the caution above). After a thorough interior cleaning, the appropriate-sized bladder can be placed in the tank cavity. Filler hose, fuel lines and vents can be routed as needed through the top or sides of the aluminum tank using standard pipe fittings with the appropriate hose or rigid line connected as needed on either side. The access hatch in the tank should be re-installed as an inspection/maintenance hatch. The bladder will be protected from abrasion and fire damage by the aluminum tank.

Because bladder tanks are made of fabric, the space enclosing them must be free of sharp points and roughness that may puncture or abrade the bladder. The bladder must also be secured in place to ensure proper expansion and collapse as the bag fills and empties.