I assume you mean my rear gasoline tank? Nothing is going to be as safe as a mid-ship mounted underbody OEM tank. Those things are tested to hell and back. If there was a factory (or aftermarket) underbody tank for my van, I'd probably look into that first. But there isn't, so I bring the jerry when I can reasonably presume that the extra 5gallons might be needed (longer distances or questionable fuel availability). I feel better about NATO steel cans and real Scepter MFCs than I would about a "consumer" grade can, but the reality is that if I get rear-ended by a big enough truck, a rupture is not out of possibility.
Regarding the rear propane tank, I'm trading off the risk of rear-end impact for the portable tank vs. damaging an underbody tank going over rocks, etc.
As with the gas jerry, weigh your own risks accordingly.