Mike, FWIW, if you look at any Euro-van installs, they've all got their bottles inboard too. The gas is heavier than air and as long as the compartment the bottle in kept in is vented out the bottom in some capacity it meets, I would think, are some strict Euro-guidelines on safety and such. If you do a little search/homework on a propane install by one of our 'over-there' van-builders I'm sure you can find and follow some good, sound advice and not be burden with a tailgate or underbody tank. I assume when I say, RVIA must adhere to whatever, hence we get what we see.
As an example, some time ago I had an AWD-minivan used as my mountainbike/ski basestation with some buddies that did somewhat the same, cheap, means of over-nighting while we rode. I used a Mr. Heater Buddy and a 5-foot hose. To vent the heater, as a byproduct of propane is water vapor, I place it against the passenger slider door and with it ajar, and still lockable, it was vented from the bottom for oxygen and out the top to vent. Host ran to the 'trunk' I built; simply a partitioned wall across the back, window high, and this is where I kept the 20lb tank. It was vapor-safe by having built a Reflectix 'bowl' about the van. From the driver door back, around the trunk wall, and up to the slider door, including floor. One big foil-tape unit to keep out drafts and to seal against any propane-in the rear that was also vented if in the event that gas actually filled the 'trunk' area, vented out the rear-heater vent slats. Had a cheap 6' wicker couch to be off the floor and I recall it being actually zero degrees out and me doing OK in gym shorts/shirt heading my my 40 degree bag at night, and I'm still here alive to state it. Never had a gas or vent issue for this temp-setup. I did test it all with a soapy solution and had appropriate battery-powered alarms; both carbon monoxide and for propane of course. Little long winded here but that much heater did that capacity of heating just fine and did not take any undue risk I feel after and thorough read on what to do and not do, and a defense against fault otherwise. I plan the Dickinson for my 130" build still.