If you read the install guide, I think the call for 3' of separation. Which is why I was surprised when I saw that cool single piece vent fitting you are looking for. No, not to hot, and not all that "enclosed" either. These Propex furnaces are amazingly efficient. There is not a lot of air leaving that exhaust tube. I made the hole in the plywood bigger than necessary to clear the tube, and made sure the supplied silicone sleeves were in that space so as to minimize how hot the tubes were that might be in contact with the wood. Sealed the remaining gap with red high temp silicone designed for automotive use. No issues so far in the two camper builds I did with Propex heaters.
good to hear. yes, i liked the looks of the propex vent, but i also like your solution - less stuff going through the exposed exterior. and upon reading the install guide again, i think it helps to have the inlet and outlet a bit farther apart. very much appreciate your input.