There was a thread not too long ago reminding us all that propane bottles need to be stowed in an accessible area (no locked compartments) and IIRC at a certain size the bottle needs to be completely vented to the outside (feel free to check me on this detail). That is way you usually see them secured openly on the tongue, with perhaps just a simple shroud to dress them up. If in the back of a p/u truck, strap securely so it can't move. For passenger car, there is some preferred travel cage arrangement, that the propane industry give some details on, that allows you to carry on the floor behind the front seats, but I think that is restricted to short distances from supplier to home, and they advise keeping a window down, no storage in the vehicle.
Perhaps someone can help remember the thread where these specs were cited, as my memory may not be concise.
IMNSHO, common sense (although not always too common) suggests that a secured bottle (bolted, clamped and/or locked at the base flange) mounted outside and in a location where it is less likely to become damaged in an accident (on the tongue... I'm not a fan of off road style fender mounts) would be the best practice.