You CAN buy just one Thule or Yakima rack, and then a truck-bed rack for the back and it works just fine. Definitely load the fishing kayak upside down - most SOTs (sit-on-tops) haul the best that way. You can use either the foam blocks designed for canoes; or "pool noodles" on the crossbars. Otherwise, you'll need saddles/cradles or something to cup the hull and a cockpit cover for a sit-in.
Regarding a 2-person on the back of a 5th wheel?? Probably not; most are 12' long or longer. I personally bought a Sea Eagle Fast Track inflatable kayak that can be paddled single or double for our 5th wheel - very easy to inflate and paddle!