That rusty area is part of the flush mechanism. The rust would have been caused by a long term leak. Those flush valves are a high failure rate item and are readily available at RV dealers and Camping World. Removing the toilet requires two rusty bolts be removed, a plastic bag put over the hole, and the water pipe removed.
Here is a link to my '94 Bounder repair blog, Toilet Issues
. There is a picture of that valve your picture is showing. The pic at my blog shows it after removal. One time my toilet had a drip-drip-drip that turned out was just the water connection that only needed tightening on the flush valve. A 1/4 turn with a set of channel lock pliers was all it took.
The plastic valve is usually the leaker but there are hoses and clamps under there that should be inspected...so remove the toilet to get a look. Rust is not harmful to that mechanism for the most part.