Oh boy, does this sound familiar!
We went through the exact same thing for over a year. First troubleshooting round they found a bad/intermittent flame sensor that was causing the flame out. Worked for less than a month then started doing the same thing...diesel burner would run for a while then fault with a flame out. Troubleshooting began agin with Newmar and Oasis with everyone scratching their heads because the flame sensor had been changed. Over the course of a year we tried everything, including running the unit off of an external fuel tank, nothing worked. Finally I told Newmar and Oasis that I was going to demand that they completely replace the system. Oasis wanted to try the "shotgun" troubleshooting technique first (throw parts at it until it works). The sent me a set of every replaceable component for the system. First part I replaced was the flame sensor, since it is the easiest to change.....fixed it right up, no problems in the last year.
Long story short....I'd have him replace the flame sensor, it's easy to do.
Lee, Leslie and Ellie, the wonder dog
2014 Ventana 4369
2016 Wrangler Unlimited Hard Rock toad, Blue Ox, Air Force One