It might be that the dumpster is just too much for them. If they are willing to work a little at a time, here are some possible tips:
1. Clear out the car trunk. You don't want the donation site to take a toolbox or something.
2. Put a couple of small to medium sized boxes in your parent's trunk. They can add items a bit at a time, avoiding the whole trying to pick up and carry a heavy box scenario.
3. Clothes? Use old Kohls, Penneys or other plastic bags. Load them in, tie off, drop in trunk.
4. Set aside a box or two in the garage for the trash. The folks who can easily get to a dumpster could pick up that stuff, leaving the box for the next round of items.
Then drive to the donation place, let them unload the trunk, it's a pretty easy way to get rid of the stuff that is still usable.
5. Spend some time shredding. Do it yourself, or call your bank/credit union and find out when they are having a shred day. Start with easy stuff, like receipts from utilities 10 years ago and work your way together to more current paperwork.
6. Maybe set a time limit for each time you work together...like, we'll tidy up the files for an hour, and then do something like take a walk, grab lunch, or what have you.
I understand your parents may not be willing to ever go through these steps, but you might check in now and then and try to start the process. Too much stuff can be exhausting, even though they may not realize it now.