Basic options:
- Rack on the back: Downside, it tends to be bouncy and the typical sewer pipe tube is a little iffy for holding it.
- Back of the truck: Downside, less convient if you want to take the truck while the wife stays at the trailer.
- In a dedicated storage space: Eats up valuable storage space.
- Mount on top of pin box: Without electric start, may be hard to start if you are short.
I would recommend against just sticking it in any old storage space. As you noted, there are potential spark producing items that could set off any fumes that accumulate. Not to mention, a good probability, the odor makes it's way into the living space.