I looked into it....
And basically what has already been said is correct.
You only lose boot space if you keep the spare wheel.
They produce donut shaped fuel tanks for the LPG which replace the spare wheel. Trouble is most people don't want to lose the spare, so that just rolls around in the boot.
The thing is, are you going to do enough miles to warrant the conversion. I would reckon that you would have to do at least 15,000 miles per year for 3 years in a Bravo to recoup the intital outlay.