Congrats on the new rig! If you know the size of the king bed, you have all the info you need. If it's shorter than standard, Camping World sells sheets that will fit.
We have a short queen, and still use regular queen sheets from Wal-Mart. They are a bit bulky, but not much, and that can be solved by using a couple of sets of the sheet holders (I don't know what they're called) to take up the slack under the bed. Problem solved. That way you can use off-the-rack sheets.