Heidigrrrl,
I'm sorry I did not notice your question. By disposing of it properly, I pour down a drain that goes to a waste water treatment plant.
Yes the Propylene glycol that is in RV antifreeze is biodegradable. And if you spill a small amount on the ground or concrete all you have to do is dilute it with a large volume of water.
For the volumes you would collect during de-winterizing, I don't want to pour that on the ground or put it in a storm water drain where it will not be treated.
Here are the instructions from one Material Safety Data sheet.
Diluted aqueous waste may biodegrade, but avoid overloading plant biomass and assure effluent complies with applicable regulations.
Hope this helps.
Ruide