Hi neighbor! I'm quite familiar with Wattsburg, I've traveled through there many times while working for the Dept. of Potholes.
I know some manufactures are against using undercoating, especially on the flooring because it is better to leave the material exposed to the air. On the frame and other metal, cleaning, priming, and painting any rusty spots with Rustoleum may be sufficient.
Anybody want to add to that?