You could use 12-2 w/ground cord, use a SO or SJO type jacket. This is stranded wire and much less stiff than Romex. Since this is stranded wires, have to either have the proper termination where you are landing it on both ends or use a insulated crimp ring terminal on each end.
Most slides outs have SO or SJO cord when they need to get power into the slide and need something that flexes with movement.
Note that runs of less than 50' of wiring that is rated for the maximum amperage don't occur that much voltage drop on 120AC. Unless you are loading at the 15AMP level for the 14-2 forever you will never notice.
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