I have the same 8551 reefer as you.
I first encountered the same non-cooling problems with propane as you a few years ago, and have posted a few responses to this forum.
Here are my observations - between 20*C and 25*C, the fridge can work like a champ, but if the temp goes up closer to 30*C, then you will have >5*C fridge temps, whether you are on propane or AC.
I have experienced trips where the fridge refused to cool on propane, even on cooler ambient temps. Most times the fridge lights, but refuses to cool in warm weather. I have always been able to fix this by removing and soaking the burner in alcohol. I usually find a bunch of brown rusty grit blocking the burner orifice. Cleaning the burner usually gives me a solid blue hot flame that can make the reefer work. My other theory is that rough roads can loosen grit in the propane line. Your fridge can work fine on your driveway, but after a day's drive, some bit of grit has plugged up the fridge burner.
Cleaning the fridge propane burner has become part of our pre-departure checklist.
Kirk & Shelley
Surrey, Beautiful BC, Canada