I am not familiar with the 2012 LS Avalanche, so I am just venturing a guess or two here.
When you checked for fuses, was there one in the under hood fuse box for RTD or Real Time Dampening?
On my 2004 Cadillac Escalade EXT, it has the RPO code Z55 Chassis Package Bi-State, Real Time Damping in addition to the RPO code G69 Level Control Auto, Air, HD for the air shock system.
All four of my OEM shocks were part of the RTD system, but only the two rear shocks were part of the leveling system.
The RPO code Z55 refers to the truck having shocks at all four corners that are connected electrically to the truck's computer.
You should see some sort of wiring connector plugged into the upper portion of the shock.
The computer is able to adjust valving inside the shock according to need to maintain the desired ride firmness at all times.
It may be possible that your truck was factory equipped with those type of shocks and a previous owner replaced them with standard shocks because of the cost of the OEM replacements.
That is not all that uncommon since the RTD shocks tend to be much more expensive.
What some owners do not know or understand, when replacing the OEM shocks, they must also install special resistors at the point where the wiring pigtail normally plugs into each OEM shock.
If the computer does not see the normal OEM shock or the resistor, it senses that something is wrong with the system and it will command the error message you are seeing to be displayed.
You should be able to locate the wiring pigtail near each of the four shocks.
Hopefully, the previous owner did not just cut the pigtail off.
If the pigtail is there, you have a couple of options.
Purchase and install the correct OEM shocks and plug the pigtail into those shocks and regain the RTD function on your truck.
Or, locate and install the correct resistors at each wiring pigtail and secure those under the truck so that the computer sees what it is looking for and quits displaying the error message.
Or, if your truck is so equipped, like older models of these trucks are, if there is an RTD fuse in the under hood fuse box, with the truck turned off, remove that fuse then disconnect the battery and after a minute or so hook the battery back up and start the truck, leaving the RTD fuse out.
On older models of these trucks, that procedure will cause the computer to stop looking for the RTD system and your error light will not be displayed any more.
Anyway, these are just a few thoughts I had that may or may not apply in your situation.
Let us know what you come up with.