The floor support beams are square tubular steel, as shown in the diagram below...
The tubular steel is unprotected in most areas, however at the seams where the floor meets the wall, they had sprayed foam insulation to protect against water. However the foam has not stuck very well over the years and so moisture has gotten under the foam and caused the steel in those areas to become quite rusty. Also, water has gotten between the seam where the wall meets the floor, and when driving in the rain there are areas where it gets wet and then takes forever to dry.
Most of the steel is still structurally strong, with just a bit of surface rust. However two areas are weak, although not in an area where it's going to cause a problem. So the key is to make sure it doesn't deteriorate further.
During some of my working life, I used to be a licenced auto body technician and painter. So thankfully, I kinda know what I'm doing. Sure is messy, dirty work though so lying under the motorhome with rust flakes dropping in my face reminded me why I won't ever do that line of work ever again!
Removing the foam to access the rusty tubular floor frame.
So the procedure is to coat the visible steel with "Must for Rust" (for some stupid reason, not available in Canada...I bought mine when I dropped our son off at school in New York a couple of weeks ago) and then paint with black Tremclad rust paint. Then, reseal those areas with more spray foam insulation.
Overall, Sherman is in pretty good shape for a 14 year old motorhome. We think that with proper regular maintenance, Sherman will be around for quite a few more years. Only 50,000 miles (80,000 kms) on the odometer, so the engine is only just broken in.