You're unlikely to get more than 5-7 years from the batteries even stored as your propose.
This link describes many factors that contribute to shortened or lengthened life and notes
"Recent
industry experience indicates that a 4 to 7 year VRLA battery life is more likely"
http://www.power-thru.com/documents/The%20Truth%20About%20Batteries%20-%20POWERTHRU%20White%20Paper.pdf
...
"A European study of over 1,000 installations, of various system voltages and cell
capacities, containing about 35,000 cells concluded that VRLA batteries require
replacement after 5 to 8 years of operation. The shorter lifetime (5 years) was
associated with batteries operating at 110 V or higher system voltages. The
longer lifetime (up to 8 years) was applicable to better quality batteries and those
of lower system voltages. Absorbed Glass Matt-type cells demonstrated a higher
failure rate than did gel-type cells."
These are for commercial quality batteries, it's unclear what you have, though if you know and can refer to the datasheet you'll get a more specific answer. The batteries in the study are generally used in UPSs so are kept on float charge, as you propose, except for a drain 1-2 times a year on test.
This is a link to a VRLA battery with a 20 year design life on float charge, so it is possible but again extremely unlikely this is the battery you have.
http://gemenergy.com.au/wp-content/uploads/2017/01/Section-26_05-2015-02-Absolyte-GP-Industrial-Batteries-Overview.pdf
