It is happening, Long Range America is building Fuel tanks for both Grenadiers

[LeeroyJ]

I have carried jerry cans both inside and outside (roof rack) of the vehicle. I'm with Peter, no reason to not carry inside other than space.

I would still install an extra tank under the truck if I didn't live in CA where they frown upon such things...

I've carried jerry cans inside the vehicle for many thousands of off-road miles. That does not mean it is a safe practice. It is not. If that's within your risk tolerance then continue to do so, but that does not make it a safe practice.