Back in the early '70's my dad put a dual fuel LP system on our '67 Chevy pickup with 327 small block. From what I remember, it used the regular gasoline carb with a shut-off solenoid for the gasoline supply, a shut-off for the LP supply, a shut-off for the air cleaner intake, and a "mixer" chamber that mixed the LP with air so that the air/fuel ratio was correct for the carb to meter, as well as a hundred gallon LP tank in the bed, the appropriate hoses, and a 3 position switch (LP-off-gasoline) on the dash. That's all I remember about the system, thought there may have been more to it.
In Texas, we had to have a tax stamp sticker on the vehicle in order to buy propane as a motor fuel. If you're filling tanks on a RV, how are they gonna know what the LP is for? We would run LP until the engine started to sputter, then switch over to gas and start looking for an LP dealer. Overall range with both tanks full was about 1,200 miles. Once my dad had a friend along on a long trip who brought a 12-pack to drink in the passenger seat (legal in Texas back in the day). Eventually he turned to my dad and asked "Don't you EVER stop for gas?" Poor guys bladder had had enough I guess.
Being able to shut off the fuel was handy on dates, but I think I'll leave that to your imagination as to why.