Read post #8 in this thread, it'll get you started on researching how the "unit installed" building process works... Seems that Keystone's line crews install one duct assembly per run (whether the run is 3' or 33'.....
http://www.keystonerv.org/forums/sho...highlight=duct
It sounds like the duct work was restricting airflow and your furnace "high temp" switch was shutting down the furnace to prevent an overheat condition. Once the "high temp" switch reset, the furnace would reignite and continue heating the RV until the "high temp" switch called for shutdown due to overtemp again.
In normal operation, the furnace should continue to run until the thermostat is satisfied at the temp setting, then the furnace burner shuts down, fan continues to run until the furnace is cool, then the heat system remains "off" until the thermostat calls for heat again. Initially, there may be one or two cycles with short timing between heat cycles due to cold walls, furniture, etc. Once the interior is up to temp, then cycling should only occur when the thermostat calls for heat.