Forklift "tanks" are actually cylinders (cylinders are removable; tanks are built-in). But, they have one distinct difference from your standard DOT cylinders: They are designed to be mounted horizontally!
In other words, no, you can't use one on your Airstream. The amount of modification you'd have to do to your trailer in order to mount the cylinder makes it impractical.
Engineering: Finding complex solutions to simple problems you didn't even know you had.