Barring something being really wrong or a control board problem, it's the thermostat that controls the flame burning time. Check for any furnace warmed air being blown at or being redirected towards the thermostat area. I've seen it happen in RV's several times that warmed air being blown towards the thermostat will tell the bimetal it's now warm enough to shut the gas flow off, so it does.
When first warming a cold RV inside, turn the thermostat up at least 5 degrees higher than your final temp desired because all the interior walls and objects are still cold and the cold coming off the still cold inside surfaces and objects cause the furnace to run for short period cycles until all the interior surfaces and objects warms also. Once equal warmth is acheived, turn the thermostat back down to your desired living level.
This same cycling occurs in a cold home temp set to 55 degrees that's being brought back to 72 degrees and we have it happen every year when we return from our couple winter months in Florida. Once the home's total interior gets up to temp, the quick short cycling quits.
