Don't remember where I got them but "eraser wheels" attached to a drill were effective for me. Got three of them off the internet and used them to remove severely cracked decals that had been exposed to Texas sun over a couple of summers.
These were solid discs of eraser, gum colored. You hold it up to the decal and it heats it up and 'erases' it. Some material from the wheel is sacrificed in the process, like a pencil eraser, but it is effective.
A quick search shows similar products available that are fluted. Don't know if they are better, but they sure cost a lot more. I got the original three disk, complete with shanks to be ready for a drill, for about 18 bucks. One disk is good for about a square yard of decals which will take about an hour..