Propane tank transportation safety?

Is it unsafe to carry a five gallon propane tank in an enclosed pickup camper shell? I left my other tank at home this summer and used disposables for my bbq because my wife was concerned about transporting a propane tank in the shell.

It Would be nice to have another five gallon tank on board to be able to switch out the trailer tank if it ran out at a campground. I ended up just buying an inline LPG gauge so I could tell how much gas I had left on the rPod tank.

Is she being overly cautious or is this an unsafe practice?

Thanks in advance for any input.

Instead since I was curious what regulations actually apply to the private transport of propane cylinders for personal use (I say private because the USDOT regulates commercial transport but that doesn't apply to us), and what they require. There are DOT regs which apply to the cylinder construction, filling, certification, and recertification but those are not the topic here. 

Also, I ignored any "rules" being promulgated in presentations by various players in the lpg industry. Those are not regulatory. I also excluded the regs controlling transport of cylinders which are part of the RV. Those are in NFPA 1192 and apply to the pod itself, including any mods you do to them. 

But so far the only standard I found that applies to personal transportation of propane in the tow vehicle is NFPA 58. To be clear, the NFPA standards in themselves are not regulatory either, they have to be adopted by local authorities having jurisdiction to become mandatory. Most AHJ's do adopt the NFPA standards, but they might be a cycle or two out of date or have local amendments. NC has apparently adopted the latest cycle of NFPA 58.

To paraphrase 9.3.2 of NFPA 58 you apparently can transport up to 420 lbs of propane in an open pickup bed and 90 lbs in an enclosed vehicle, which would include cars, suvs, or enclosed pickup caps. There is a max of 2 cylinders if its an enclosed vehicle. In all cases the cylinders must be positioned on flat floors or in racks and secured in position. If the cylinders are sized for less than 45 lbs propane then they can be loaded in any position. No ventilation requirements are listed. Disclaimer: this is not a formal statement of the requirements nor a recommendation, just my attempt to try to determine what regulations allow you to do and restrict you from doing.  

So there it is so far. I'm still going to look for other regs that might apply, perhaps there are local county or city level ones in your areas, I haven't found any where I live yet. 

And to keep things nice and explosive, we also occasionally carry a can of gasoline in a steel UI-50-FS-Type 1 container, sometimes a couple.  But then, we have a rapidly decreasing 20 gallons of gasoline in a tank under the truck too.  All a bomb waiting to go off.  It's a good thing we don't smoke.

I would prefer diesel as a fuel, but my truck came with a gasoline engine and it's not practical to change it.  I did that once with a boat, changing the engine to diesel that is.  I sprang a leak in a 20 gallon tank of gas at sea and it was a harrowing trip home with gasoline draining into the bildge.  I replaced both the tank and the engine.