I had a pretty much rust free F150 before this and actually hosed/washed the chassis down every time I got back from using it, within year and a half there as brown rust spots coming through, I was gutted.
I will try and limit the use during these times but that means I'm in the GC, fortunately I've smothered that one in waxoyl
You can coat the chassis with something like Fluid Film. That stuff when applied liberally several times a year will keep any rust from occurring. It works very, very well!