Depending on the model and its size, Crutchfield has one or two replacement RV radios that are nonstandard (larger than usual double-DIN car radios) and claim to replace certain Concertone models. They are not low-cost options, unfortunately.
If the radio doesn't work after testing, if I were in your shoes I would either leave it disconnected and sell with the caveat that the radio doesn't work, or else replace it with some standard sized car radio and put some sort of a filler panel or cubby or something to fill the remaining space.