If your set up is similar to Dave W's then obviously you'll need some kind of step down/conditioning to use your radio on AC - probably more trouble than it's worth.
Also depending on your set up (particularly your trailer electrical
connector) - if you have 7-pin coming into the trailer you could run it off of the tow vehicle power. It would have to be fused and most likely the stereo has a fuse in it anyway. The other issue of course is a) does the tow vehicle supply power when it's shut off and b) running down the tow vehicle battery...
I would think that you could find a cheap, off-the-shelf a/c - d/c power supply strong enough for the stereo (from digi-key or amazon) as a stop gap until you decide on a new converter.
If you're unsure of whether the radio itself even works - find a pin-out/installation instructions for the stereo and hook it up to the battery on your tow vehicle.