Short of grinding off the rust spots and taking the motorhome's rear bumper down to bare metal, I haven't been able to find any way of removing the rust. The Napa rust dissolving gel was a complete bust. In fact I seriously feel now as though I wasted my money in buying it. What I finally did was take a stiff wire brush to the entire bumper and knocked off the rust flakes and anything else loose that was on it, then painted over it with Rust-Oleum semi-gloss enamel. I probably should have put a primer coat on it first.
The upright just above the trailer hitch is a Kenco tire carrier. Instructions for installing it can be found here.