RV furnaces are designed to expel the heat through the ducts. If one of the ducts becomes blocked, the remaining ducts will work a little harder to get the job done. In extreme cases the fire in the furnace shuts off and the fan continues to push the warm air through whatever ducts remain.
Furnace hot air is not going to burn anything to catch on fire. I accidentally block one of my vents on a regular basis with a vinyl footrest and pillow, the hot air then gets exhausted through the remaining vent and has never overheated to automatically shut down.
