Most FLA or AGM batteries have shortened lifespans due to over discharge, lack of full and complete regular charging or poor maintenance. The stock converter chargers in many RV's don't do a great job getting batteries to 100% and many don't do equalization charges. In addition the standard battery monitors give poor information as to state of charge so you are basically guessing leading to the batteries easily becoming discharged too low.
If you want a battery that will last many years go with lithium and also install a true shunt based battery monitor.