[JRTJH]

If the radio is removed from the trailer wiring, you can always connect it to a battery with a "make-shift" power cable. Install a 15 amp fuse in that power cable positive wire. If it blows the fuse, you'll know that it is definitely the radio since there'll only be the radio, two wires and a battery involved. If it doesn't blow the 15 amp fuse in that wiring situation, I'd contact the radio manufacturer for advice.

My guess is that when you momentarily connected the battery in reverse polarity you damaged the power circuits in the radio and it now is putting a larger demand on the converter. I'd guess that it's only a matter of time until you "let the smoke out of the radio" and it will stop working completely. Strange thing about electronics, once the smoke is all used up, they all tend to stop working. Magic smoke is hard to come by, so a replacement radio is probably in your future.