Melle.17, deaing w/ any rust issues now is way less costly(time sweat and $$) than facing it on down the road
a product called Ospho has been a staple for this issue for many years
http://ospho.comscuff the affected area(s) w/ a Scotch-Brite pad to remove all scale and simply paint it on
it turns black and can then be painted w/ a rust inhibiting product like Rustoleum(brush or spray) to help in reducing further issues
former camping trailers like yours frequently become available due to water intrusion and deterioration(rot and delamination) of non waterproof materials used in the original manufacturing process
if you wish your efforts and investment to last it is strongly suggested that you use all waterproof materials/plywood and not get caught up in the false economy of using less noble products like underayment OSB particle board and non exterior(actually labeled on the sheet) plywood
some glues are listed as "water resistant" which doesn't mean waterproof
some will tell you about their successes using the lesser products and a few do actually get away with it butt in the long run it isn't economical or prudent
GOOD LUCK W/ YOUR BUILD
many of will be following along
sw