As a person that has been homeless, I just have to say that not every person that is homeless needs to go to a shelter. I'm sure the advice was kind hearted, but a good car is really better than a shelter in 90% of cases. Many homeless people are capable and resourcefull people that do not need handouts to continue with the adventure of their lives. Now, back to propane.
Propane will leak from cylinders if they are heated too much or if they are defective. Once disconnected, and not being used, they can be tested for leaks at the service port and releif valve with soapy bubbles. I'd use sudsy dish water. As far as heating them, keep them under 90f and they won't vent assuming they are properly filled. Disposable ones that are filled at the factory are always properly filled. Larger 20# containers will be properly filled if they are new enough to have an overfill protection device. In your situation, I'd make sure you have a tank with one.
If the tanks must be heated over 90f, then they may vent some propane so they cannot be stored in an enclosed area.
I've traveled all over the US with 1# disposable cylinders in my car, in my backpack, and in my tent. They are very safe if you keep them from getting excessively hot, avoid punctures, and test for leaks before storing in an enclosed area.
