Slightly off topic, one of the ways that hollow sections/sill/chassis etc can be protected, ideally from new, is to use expanding PU foam.
Its a very well trodden path in the automotive industry, in fact for a while in the 1960,s Austin Minis had foam injected into their sills, it was in an effort to stop leaks, but the side effects were, huge increase in torsion resistance, made the whole structure stiffer, and years later when Austin recovered some these cars and cut the sills open, they had NO corrosion inside them!
There are plenty of papers on line giving more info.
If the chassis was very clean and dry inside its something that could be worth doing, you would just have to ensure all the cavities were filled.
It certainly sounds like it couldnt make it any worse!
Mark