DW and I were JUST talking about this this morning!
I would avoid the rear bumper unless you can use something mounted directly to the frame. Like those bolt-on receiver tubes? No way. I would constantly be afraid of gas cans falling off and becoming little road bombs!
Lots of people have bolted or welded receivers to their frame though, and that I would trust. You could also fabricate a rack on the tongue to go up over the battery and propane. I've seen that done before.
I wonder if you could fit a special rack there on your tongue? Something like this:
Wavian - Steel Gas Can Holder | 4WheelParts.com
I have the benefit of having a pickup, so I could store mine in the bed in a purpose-built rack. That's my current plan if/when I decide to carry extra fuel.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!