I've known it to be done to older large motorcoaches that have a heavy chassis to support the extra weight and the slideout mechanisms. They've usually also had metal framed walls or laminated walls that could be framed around openings. I don't know what would happen to the structure if one started to cut a big hole in the side of a stick-built box like many of these 1980s travel trailers.
Prices I've seen for add-on slideouts (this would be motorcoaches) run $2000 to $2500 a foot. Economics? $16,000 to add eight feet of 24" deep slideout to a RV with $4000-5000 market value?