Here is a link to a place that sells products for rust coatings/conversions/treatments, etc for chassis.
EastwoodsSome of the products can be applied right over the rust and they will convert it and seal it to help prevent more rust. Just find the product that you thinks will work out the best.
As far as jacking the motor home up, I personally would only do one end at a time to improve stability. I would not try to get the whole coach up in the air. Use your leveling jacks to get one end up as much as possible and then put two heavy duty jack stands underneath the frame for safety.
12 ton jack standsRemember the jack stands have to be on a surface that can support the weight. Gravel, dirt, grass, sand and most asphalt will not. Concrete is the best choice to support the weight on the jack stands.