poor performance of propane in my 2013 Nissan Altiman 4 cylinder

My 2013 Altima Nissan has used propane since late 2015.  It currently has
180,000 miles of which 150,000 miles has been with the combination of gas
and propane.  Of that mileage, approximately 100,000 miles has been on
propane and 50,000 has been on gas.  Currently, performance has dropped
1/3 on both gas and propane.  It is vey hard to start. My mechanic has
diagnosed two valves that are not seating when it is cold.  When the car is
warmed up, its performance is much better. I love this car for its extended
range and the fact of going 15,000 miles between oil changes. And the fact
that the mechanic says the oil looks as clean as when he put it in.  Is there any
hope for this car.  I love running the inexpensive propane.
I was told that the engine would run clean and not wear out.  I ws hoping to
get at least 300,000 miles on this car.
What do you think is the future for auto propane gas on cars or pickups?