Propane conversions were popular in the late 70s and early 80s when gas was scarce and prices were high.
We did one propane conversion at the shop on a fairly new Ford Fairmont. It ran terrible cold until the cylinders got hot. Removing the fuel tank and installing a propane tank is not for the DIY. Each vehicle is different.
You could buy a LOT of gas for what it would cost to do the propane conversion, and then you probably wouldn't like it.
Some municipalities have propane vehicles, but they were designed around propane as a fuel not as a conversion, and they have a lot more software control than back in the 70s and 80s.
I am in Australia and had an LPG system installed about three years ago in my 3 Door Rav4. My system starts on petrol and when it reaches temp, it then switches over to LPG. System runs great and my capacity is 50 usable litres of LPG. Economy only slightly below petrol. The tank is fitted behind the rear seats. I service it once a year and have not had any issues with it.
Hope this helps and feel free to ask any questions.