GhostOrchid,
Battery life is really based on amount of usage in cycles. A lightly used and well maintained AGM like Dustin's will easy make 10 years plus. The storage capacity decreases with the usage no matter the age. So, Dustin's is still working great but maybe it only has 70% of it's original capability (no idea the exact amount, just a WAG).
Of course a couple big discharges will reduce that capacity/years formula somewhat but if you are storing it properly with the maintainer then I would say you still have many years left if using it lightly the rest of the time as you describe. Now, if you are using it heavy whenever you do go out then that will affect the run time because there are more cycles on it. For example, if someone is regularly using a large DC load like a DC fridge or a CPAP machine and doing extended stays off grid, maybe with a solar panel on it that keeps charging the battery back up each day the amount of cycles on the battery will go way way way up. For someone doing this time of camping we see an AGM only lasting 3-5 years before the run time is just so short it becomes and issue.
To answer your original question though, to test the battery you would take it out of the camper and take it to an auto parts store or battery store to have it load tested. This will give you a rough idea where it is in it's life span.
Cary
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