I had a similar problem with the fridge install in my A-frame (which uses side vents for both intake and exhaust). There was no cap or diffuser on the top of the propane burner exhaust (like your photo). I used a 1/2" copper 45 degree elbow and a short piece of 1/2" copper to duct the exhaust to near the upper vent. I drilled a small hole in the elbow for a self threading metal set screw to lock the elbow in place. I soldered the 2 copper pieces together.
2019 Flagstaff T21TBHW A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time