I love all of the good questions. I contemplated many of these before choosing this path.
1. Odor. I have had zero odor issues with this setup. That makes sense to me. The only way to get an odor is if the diesel vapors can get out of the fuel system and into the air of the van. But this is a fully closed system. There is no path between the fuel system and the van's interior air. Consequently, I'm not surprised to have no odor issues.
2. Safety. This is an interesting one. I've had long conversations with a long list of folks--rv manufacturers, fuel tank manufacturers, and others--about fuel tank safety. When it comes to exterior fuel tanks, I am not aware of any laws that prohibit a person from installing one on their rig, but be aware that only a very few of the tanks out there have been crash tested and certified to meet DOT standards.
So--if a rig with a non-certified tank is involved in a crash, and that crash results in ignition of fuel from that tank, what impact might that have on liability and insurance coverage? I don't know. But that line of thinking gave me pause for thought.
Something like a RotoPax is a manufactured, accepted product. They are used everywhere. It would be darn near impossible to deny coverage or pin additional liability on a vehicle owner as a result of fuel spillage from a RotoPax in the event of a collision. That weighed in my decision.
Regarding the possibility of diesel fuel heating to its flash point, and igniting: well, it is fuel. No question about that. Diesel burns, propane explodes. By definition, we all travel with hazards to our health. Heck, being on the road is a hazard to our health. For me, the question is: what is the likelihood of that risk turning into a reality?
In my case, that RotoPax is in a well-protected location. Could it be penetrated in the event of a crash? Yes, but it would take one heck of a crash to do it, and even then, nothing would come of it unless the fuel also managed to vaporize and and the vapor came into contact with something hot enough to get it to burn. Are all of those things going to happen? Theoretically possible, but highly unlikely.
If enough happens to set my RotoPax on fire, it is probably enough to get an external RotoPax to burn, or a rear-mounted, under-floor fuel tank to light off.
Conclusion: I feel safer with my RotoPax full of diesel on my rear bulkhead than I did carrying a can of Coleman fuel, or portable, pressurized propane or butane cylinders in my rig. When I add up all of the risk factors, I'm pretty comfortable with this setup. But to each their own. One of the beauties of the van life is that we each have our own set of priorities and considerations, which all lead to different ideas and different conclusions...which leads, in turn, to the magnificent conversations we have on this forum!