Looking for Short, Used Class C with Island Bed

You generally won't find an island bed on shorter models because it would require having the rear wheel wells in the walkway around the bed, which means either having a decidedly not flat floor there or having the whole floor raised up rather higher than usual.

A few companies offer twin bed arrangements on shorter units, with a center walkway between the two beds. Possibly that could be an alternative that would work for you; possibly not. (Some further have available a filler bit available to convert the twin beds to a huge king bed with no walkway.)