Is this a rental RV? If so, options are pretty limited. Realistically, there isn't any way to safely carry gasoline because even if it is in the bathroom with the vent on, vapor can build up and ignite.
If this is your RV, and I'm assuming a class "C" motorhome, you might have one option: If you don't mind punching holes in the back of it (assuming no windows), you might be able to install a RotoPax mount kit. This would allow you to take a set of various gallon containers and have them in a place where they won't fall off (assuming the RotoPax pack mount is attached to studs or other framing.) You can get a locking handle to discourage the two legged scavengers as well.