HI - range 4.6 not reving correctly ..please help!!!

hi guys new to this so pls bare with me
i have a 1998 4.6 vogue with lpg
just done a 60 mile round trip on motorway all ok
got home turned off went inside - toasty warm
came out started car but it was if it had run out of gas and/or petrol
limped to station put some petrol in but to no availe
limped home and still seems as if run out of petrol and/or not running on all 8 and wont rev propery
also tick over has dropped hence suspension not fully up

PLEASE HELP!!!

cheers guys...