Personally, I have no concerns carrying a tank inside my truck back seat or TC, other that it being loose rolling around. I seldom carry a spare tank, but when I do, I usually put it in a plastic milk crate and put in on the back seat buckled up. It can't move.
The only restrictions I have seen to do with tunnels and such, is a couple of places I have been say to turn off your propane when in the tunnel. I am far more concerned with one of the propane lines, fittings or valves (furnace, stove, fridge) failing and filling the TC with propane. I have never had a tank to fail, but I have had lines and fittings leak for no obvious reason. Most RVs are plumbed with the propane lines outside the TC, but at some point they must go inside. I have had leaks in the furnace fitting, crack copper tubing in the water heater and also on the stove/oven.
I think propane is much safer to transport than gasoline. A rock can bounce up and hit a fuel can punching a hole. I have had cans leak due to them rubbing a hole on the platform they sit on. It is unbelievable how many of the fuel caps leak. I never had any of those problems with transporting propane.
I do transport propane tanks, not often but on occasion. I do transport gasoline for the Honda. I recently started phasing out my plastic gas containers, and now use the NATO type fuel cans. I mount my gasoline cans high in the back of the TC, to reduce the likely hood of a rear ended issue. I have considered a similar method for a propane tank, but I carry an extra tank so seldom it has not been high on my to do list.
