The L5 is not CE marked, and you will need to check with your insurance about carrying gas & using a gas appliance in your van.
Personally I prefer fitting the heater inside the van, as fitted on the back door means it'll get shaken about the place every time you open & close the door,.. also its exposed to the weather & the flame could blow out.
My heater is fitted in the van with a flue fitted to vent the fumes through the van roof. I've also 3 other roof vents to let fresh air in AND holes in the van floor to allow any leaking gas to escape (gas is heavier than air).
You can get an ignition kit from the US distributors of the L5 with the 17 minute cutout disabled. I'd highly recommend it if you do go the L5 route.
Peter Fogwill has a CE marked unit which according to him is far superior (I'll reserve judgement till I hear some impartial reviews) which comes with a venting kit etc. It should be on sale very soon.