Surface rust on primary chassis components is rarely an issue sine the rust never compromises the integrity of the metal parts due to their substantial thickness. On the other hand, rust on metal parts that are thin should be addresses since the rust can eventually eat away the component. Motorhomes normally have lots of thin-metal parts that are prone to rusting. These parts should ideally be cleaned with a putty knife and a wire brush or a wire wheel attached to a drill. They should then be wiped clean, primed, and painted with a good "rust'proof" enamel. Then, all you need to do is keep an eye on them every couple of years.
In addition, Wal-Mart sells cans of rubberized paint that is used by body shops. I can't attest to its effectiveness but supposedly, it prevents rusting for a while. In any case, you would still need to prep the surface as described above. Best of luck.
2004 Winnebago Brave 34D with the usual add-ons