Professional rust removal gets all the rust out of the rust pitting
unless it is very severe. Extremely valuable cars and other equipment are
run through this process.
I would not recommend removing the frame of a RV just to zinc coat it,
but if one was already doing a body-off restoration it is worth
considering.
Much better is actual galvanizing, which is electroplating with zinc, of
a brand new frame. It is not so thick, therefore does not add so much
weight, and adheres better to the steel. Many boat trailers are
galvanized after welding into assembly. As long as the zinc is not broken
through (scratched) it will last many years. Look at old electric
transmission towers and you rarely will see any rust even in coastal
areas.
More and more motor vehicles are galvanized at least in part. My Chevy
van has a galvanized body, except for the roof. It was a rent-a-van in
Indiana. It was exposed to salt or even worse perhaps calcium chloride
(calcium salt) as is evidenced by the rust on the frame and chassis
parts. The body has no rust inside or out, even where it has been thumped
and the paint broken.
Or you could just have a new frame made of aluminum! Weldable alloys are
as strong as mild steel. If one is willing to spend enough money you can
get almost anything done. Use the old frame for a utility trailer, or
sell it to someone whose frame is totally shot.
As far as closed hollow spaces, if they are truly closed there will be no
significant rust inside and would not need coating.
Al G.