What is the average life of an RV house battery?

["Christopher45103" <blickcd@xxxx>]

While checking the water level on my batteries I noticed they were 
almost three years old. Note that I bought the trailer eight months 
ago and check the water level every few months.

These are RV / marine 12 volt batteries, and I can't remember the 
brand name if that is important. Based on the condition of the 
trailer in general, the previous owner most likely kept them topped 
off and properly charged.

How long can I expect them to last?

Christopher