For a small amount of drag, the board will be the cheapest and easiest to do. A couple of pieces of 2"x 12" x 2' will get you a lot of lift at the low point and are easy to manage.
We had a 2004 Fleetwood Terra that dragged and boards didn't help. I added the air bags to the rear with the optional pump and was able to inflate the bags from the dash when I got home. The added benefit was that I was able to level the coach from side to side and front to rear. Surprisingly, after the bags were installed and everything set to level, the coach road/handled much better.
Lastly, those casters you posted will be torn off the first time you try. There is a tremendous amount of force both downward and sideways when dragging the rear end. I disagree that casters do damage to the frame. I would rather have my frame roll through a rub spot than drag. With that said, it's important that when you install casters or a wheel, that you DON'T make the condition worse by extending the frame even more with the height of the casters.
My neighbor has the double drag bars, about 4' apart, on his Fleetwood Fiesta. They're the large loop type. He added a solid square tube to the inside of the drag bars that was wider than they were. On the outside of the tube he added the solid drag wheel. What's important here is the wheels only extended about a 1/4" - 1/2" lower than the drag bars.
This wheel was mounted in just the correct position to keep my tow bar from hitting the ground when traversing steep driveways. It worked great for over 9 years when I sold the coach.