[usrusr]

Uncompressed propane would have simply been called Propangas I think. That term had been established well enough in the second half of the century and while the Zeppelin-era was before that, I doubt that the term changed much.

But if you aim for a buoyancy-neutral fuel, you can just add hydrogen to the mix, as little or much as you need. That custom blend then would be precisely the thing you don't invent a creative name for but just use a color code ("the blue one").