Battery Life Span?

My current Interstate G27 is now in it's 10th season and it continues to power my tongue jack, power awning, and slide just fine. We don't typically dry camp but I do use it on occasions when we may lose campsite shore power to run my 1000 watt PSW inverter which in turn powers our toaster, coffee maker, wife's hair dryer (on low), and a couple of 120 vac fans. Considering the age of this battery it doesn't owe me anything but it probably is time to upgrade. However, instead of replacing it I'll likely just leave it where it is on the tongue and install a pair of G31 AGM batteries in the front pass through storage compartment and run them through a second Blue Sea m-Series battery switch so I can alternate between the old G27 (for as long as it may continue to live) and the new set of G31s. Eventually it will die, at which point I'd either replace it with a G27 AGM or if the dual G31s seem to suffice just not bother replacing the G27 and use the empty battery box for additional storage.