It's hard for us to say whether the 'strong' propane smell is normal or not.
Most R/V propane appliances do emit a bit of a smell but if you believe something is amiss on your furnace, please get it checked.
As for the ducts... it is not uncommon for shorter run ducts to have way more air flow than longer ducts. Add to that the possibility one or more of the longer ducts was crushed during assembly (common on all makes) and the fact that most of these ducts are simply flex hose and simply don't flow well on long runs.
Adding adjustable registers to some of the shorter runs may help. Just be sure you don't decrease the total airflow output of the furnace to cause overheating.