On that tankless water heater: how many btu's? what gas line size did you run? does the regulator have enough capacity for that many btu's? How big is your propane tank? I'd also agree that vent needs to be above the roof, not only because of heat damage to your box, but to draft properly. not sure that is going to work halfway up between the truck and the box. Only way to find that out is get it hot and running and get some smoke and make sure you have good flow pulling air into the flue and up the pipe. If not, it will create more heat, and a potential carbon monoxide problem. Admittedly CO probably not a big deal outside, but still.
And you're not a real man until you go out in your underwear at 2 am in the rain to roll up your manual awning. lol.