I have had mine mounted inside, its not a big deal if you have good cans. You should vent them every once in awhile. The inside of my truck hardly ever smelled like fuel, but also no carpet or anything in the back to soak it up either. If it did have any smell it was no more then if you had a rag in the truck that had fuel on it at one point. I also only filled them 3/4 of the way full.
I moved them to a bumper style rack awhile back due to re-organizing the back of my truck. This bumper had a spare tire on it at one point.
They are easier to mess with on the outside for sure, however if your careful I think you could run them on the inside too without issues. The cans stay nicer on the inside and are less likely for theft.