I would expect that as long as the corn has the RR trait, you should be able to kill potatoes with Roundup.  You will need to let the potatoes get a bit of size, but Roundup show kill them.

I was thinking about herbicides used on potatoes. However, planting corn in the year after potatoes crop is grown is a common practice. So, I would expect herbicides used on potatoes should not be a concern.

I would expect soil potassium levels would be high.  However, I won't presume the soil OM level is high. When growing potatoes, they tend to till the soil, and that would burn up some of the O.M.  So, I would not presume soil OM to be high.