I've often wondered about houses that have lpg gas heating with the big green lpg tank out the back, and the feasibility of 'T-ing off' that tank to an lpg pump for the car. 
It's quite easy, apparently.......

Yes, what could possibly go wrong!! :

Not much realy, as long as you are careful....
is it, in really, any different to me conneting a gas bbq to the caravan (Static) system.......

Problem is, you need to take off liquid at high pressure and pump it into the car, rather than vapour at low pressure, so it needs to be from a different point on the tank. I'd say it'd be a challenge to do it without anyone noticing when it's time to fill the tank.
You can also add a fill point legitimately, but then it has to be god knows how many metres from the nearest building and you have to make a declaration and pay road fuel duty on whatever has gone in the car, at which point it becomes about the same price as at the garage (if not a lot more).