Ive had heaters with flue's & without.
When i first got a lpg heater, L5, about 5 or 6 years ago i had it mounted on the side wall of the van as near to the back doors as possible, with no flue
Even though the heater worked fine, i found the whole van got really wet with condensation, even the window screen, as i had no bulkhead in that van.
I also had a couple of heaters freeze and split in the really cold weather when i had forgot to drain it or put a heater in over night.
Now i have my heater mounted on the front of my tank in the centre of the van.
Ive not had it freeze as i believe theres a big air gap from the outer van walls.
I also have a flue which goes straight up and out of the roof, now i get minimal condensation.
I know its not what you asked, but i thought i would share my 5-6 years of experience of using lpg heaters.