Serious rust under carriage to point of full removal - HELP

Hello. I have already sunk a lot of money into the maintenance issues of this coach and I really really like the floor plan and the RV is running great right now. I could buy another used one but the devil I know is better than the devil I don't know. I never checked under the RV when I bought it (stupid I know) but when I crawled under the area from the front wheels to the back wheels has layers that you can literally peel off. The joists also feel soft. This entire area are holding the basement up. Thoughts please.