I have an LPG heater in my van, but I'm still going to 2nd what Andy is saying. Without proper ventilation these heaters can be deadly,.. thats why most new ones with electronic ignition will only run for 20 minutes at a time to stop fumes building up. I'm also not keen on heaters with pilot lights for the same reason. With electronic ignition, if the flame blows out the gas is cut off,... if your pilot blows out does the gas keep flowing?

Do you run the heater with the cover off? I'd strongly recommend putting the cover back on as it will reduce the chances of any of those delicate internal workings getting broken.

In my setup the heater is vented through the van roof. I also have an additional 3 mushroom vents in the roof and floor vents to make sure any fumes/gas leaks can escape.

I'd also say that having the heater so close to the van roof could mean the heat damages the paintwork over time,.. a flue and vent would fix this.

just a thought,would it help if you fitted the heater to the inside of the rear door.you would have to leave the door open when you when working,but it might work?