I am toying with the idea of a dual fuel setup (there is a
guy in the UK that has kits for air cooled vans). But to
make this worthwhile at all I need more propane capacity,
well beyond just adding a second standard sized westy tank
under the sliding door (No I dont want to put the tank
inside the van, nor do I want a larger or second gas tank).

Must a propane tank be cylindrical to hold the gas safely?
What kind of PSI do they hold? My scuba tanks are rated for
3000 PSI and I assume they too are cylindrical for
engineering reasons.

In an ideal world, laws of physics aside (OK maybe the sides
could be rounded), I would love to get a tank that was only
about 4 to 6" tall x 12" wide x 36" long. This would fit
neatly under each sliding door.

What about the possibility of putting two 6" tanks side by
side between the fame rails for a total of 4 tanks? How
would you link them all together? How would you do a fill?

Just dreaming...
Chris