John, it depends on the type of battery you install. Some of the more mainstream batteries you see on the market - yes regular "deep" cycling may affect its life but in my experience, a suitably selected battery should still perform well on deep cycling for many years (but expensive !!) provided it is recharged correctly and not left in a discharged state for too long.
Interestingly, I come across a lot of people on the road in caravans having bought a relatively cheap or unsuitable new deep cycle and found it is underperforming in only a few weeks. What happens is they are discharging the battery at night when your capacity is reduced due to temp and high discharge rates and then they stick on a standard battery charger & have truly begun to destroy the battery.
In the example in this thread is that the 26Ah battery could in fact have its "100%" capacity at only 10Ah and hence if it's discharged to 10Ah regularly in this environment and assuming it's one of the more "off the shelf" type deep cycles, then yes the life could be shortened significantly as you point out. However, incorrect charging after a deep discharge is one of main life shorteners for deep cycle because many chargers are not temperature compensated and not suitable for a deep cycle battery which has a specific charging curve.
I suppose it's like how much $$ do you have
. Mount, scope, camera, then auto guiders, filters, field flatteners & oh - batteries... Ahhh there goes the budget for this year
. Just don't tell the missus
Cheers,
Darrin...