I read on another forum that a blast of compressed air on the venturi is a good fix. It seems that soot sometimes builds up and causes the propane to seep out too slowly to ignite on the first try. The next time the ignitor cycles you get the "poof" just like a gas grill sometimes does. You also need to check the burner exhaust tube for dirt dauber nests. A partial blockage of the tube can cause a flashback. The author also suggested checking for loose wires on all of the connections while you're at it.
I also read about some problems with certain combo propane/CO detectors. It's probably time to contact your RV dealer on that one
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Good Luck!