For an off-grid application, they do not spend years on float-charge, because you use them to provide power too frequently.  A typical UPS application will be (part) discharged once in a year, or less frequently than that: whenever the grid power fails.  A home power battery will be discharged whenever the house consumption exceeds the output from the generators, which is probably several times a day.

The fork-lift batteries will come under the heading of "traction batteries", which means they don't mind deep and rapid discharge (very much).  A traction battery is probably a better choice for an off-grid system, even if your equipment was originally sold as a UPS. :-)

If you used them as a conventional UPS, the only problem you might encounter is that they don't last as long as true "UPS" batteries.  So they might last only five years instead of fifteen.

What I am actually doing to the batteries I am not sure. No problems yet, but I do have wet tops. This is a dusty place off a dirt road, I have dry dust gray dust on them and shiney black tops where the dust is wet. Could be I over filled them alittle too much or I may be cooking them slowly. Been connected this way several months UPS mormally on and charging, but I have shut off power and used it as an inverter to power lights and such while working on wiring in the house and never had a problem so far.

Never toss a UPS because of dead batteries, just wire up the correct volts and use it as a Inverter :)

Heck even if you have to use some other methode of charging the batts other than the UPS charger. I'll be using wind on mine, just testing it so far this way.
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nothing to lose

I've had good luck charging 'wet' batteries to 'sealed' battery control voltages.

This is for systems not needing as much as they can make.  Example: A wet battery is charged for days at 13.7v, is about the same as a sealed battery charged for a day at 13.7v.  96% is is close to 99%. About full is about full.

Replacing sealed batteries with wet batteries, in my experience, causes no problems in oversized charging systems.

Equilization. I haven't put in many that would allow for this, even at peak power. If this is an issue and power isn't available, do it at a friends house. Wet batteries in a normal UPS set up should interchange fine.

Keep in mind for most of what I do, dead batteries replace full batteries on a solar charger. When the battery being used is low, it is swapped with the battery being charged. I have installed many kw of these systems that will never be counted in any polls.  I think this is the most 'used' type of solar PV watts in the USA today, but anyone could argue against it. But I degress.

On a boat is a good use for any of this stuff.