How about AGM batteries? If stored with a small float charger attached, shouldn't they last a pretty good while? I'm running a small solar setup using three 100 watt panels and three 80 amp-hour batteries to run my tankless gas water heater, LED lights for the bathroom, and eventually a 12v water pump.
here is what I had happen with them
I bought 2 deep cycle AGM batteries about 10 years ago, kept them on a float charger,
for the first 5 years they ran my lighting when the power failed (when I use to live in the city)
after that I charged them every few months for 4 years (neither one had ever been stored below 12.5V ever)
one of them failed about 3 years ago with a shorted cell,
about 6 months ago I put the remaining one in my truck, and it starts it just fine.
2 years ago I got a set of 10 used AGM batteries, they were pulled from service from airplanes
not quite sure how long they waited before I got them, but I think it was a year or 2, so they are likely about 4 to 6 years old now
I used them for one 60mA lamp at night, in the day I would use it to run my HF radio to check into a radio net at 11am.
and they were connected to a 100W solar panel with a charge controller to keep from overcharging,
so they were always topped off by night time even on rainy days.
from when I started using them, 2 have failed at the terminals by leaking