Next time you change your oil, save a little of the new stuff and get
a very small artist's brush. Daub oil directly onto the rust wherever
you see it, especially in seams as you describe. let it flow into the
seam. This is messy-- you'll wind up with oil streaks all over your bus.
Wipe up the excess, and let it sit for a few days, then wash. Your bus
will no longer be oil-streaked, but the rust will be stopped cold for
4-6 months. The oil gradually wears off, so you'll have to re-do this
after that time. But it won't get any rustier!

The oil locks out both water and air. It works, it's cheap, less filling,
tastes great.
I have to vouch for the above treatment.  The slow oil leak from my engine
has kept my heater boxes looking brand new :) 

Sean