The safest way to carry propane inside is in a sealed (to the inside of the vehicle) vented (to the outside of the vehicle) locker box. This makes sure when/if the pressure relief expels propane it is vented to outside the vehicle, not the inside.

You also need an explosive gas or propane alarm, like this one: MTI 20-441-WT 12V White Surface Mount Designer Series LP Gas Detector https://www.amazon.com/dp/B000FLLWGA

If you only do one of these, get the alarm.