I had a petrol for a while and it went fine. You can get 30mpg running around but try not to get into traffic or push hard on the A-roads. Best was a 36mpg over a 200mile trip but keeping to 60-65ish. Diesel will be ultimately faster and more economical for both fuel and road tax.
I am starting to think about another multi and would prefer the diesel as I will be blasting across Europe a few times in it but then again a petrol with LPG might be a goer if the right car comes up at the right price.
There is a huge amount of space under the floor designed for the tanks, Fiat fitted both LPG and CNG gas tanks here as standard.