There is a safe way to carry additional gasoline not in the living area of an RV - but it's expensive to achieve this safety.
We carry a 2 gallon version of this 5 gallon container in an exterior storage compartment of our Class C motorhome. We carry this gas for our backup small battery charging generator - but of course could also supplement our main engine tank from it in an emergency. Even using this container, the storage compartment we use is far from the open flame of the refrigerator's outside vent, far from the open flame of the water heater's outside vent, and far from the propane furnace's outside vent. All of our outside storage compartments are also lined with steel.
Here's a link:
http://www.uline.com/Product/Detail/H-1851R/Safety-Storage/Type-II-Gas-Can-5-Gallon-Red?pricode=WY777&gadtype=pla&id=71302527562&gclid=CMWTgrSF0L0CFRdsfgodo4gAkg