Some of the trucks we have put in the surplus auction at work had soft frames. A 2000 chevy 1500 4x4 W/plow, and a 97 F250 W/plow. Both frames were rotted even with the mid ship fuel tanks, as all the salt just lays there, both did not have one good rear spring hanger left on em, and the Chevy also was rotted on the boxed crossmember under the Rad.
My extensive rust bucket experience as a DOT mechanic has taught me that door bottoms and pinch weld areas of sheetmetal rot first, followed by oil pans ,diff covers, etc. Then spring hangers shortly followed by the frame. The stuff I work on sees more salt exposure than most typical rust belt cars, which is why they decay so rapidly.
PS. Any boxed frame areas seem to fare much worse than channel areas as far as rotting out.