Advice on rusted basement supports.

I just looked at a 1996 Dutch Star with a slide today. 37ft with Cummins and 6 speed. Spartan chassis. I like the inside but it did have some water stains on top but floor looked and felt good everywhere. Owner has kept it up faily good while owning in the last 4 years. Here is my problems with it, 1) The basement angle iron supports are rusted.....thru. All basement floors look good from inside, but I did not pull back any carpet. They also felt solid, but the rust is there on the bottom side! How hard to fix, replace?
2) The roof has never been resealed while he has owned it and sealant around some parts are dry and I am sure have leaked resent. But, the roof had a full coat at some time, not just the fixtures sealed. And it was not a solid feeling, a little give. Is this normal the roof of a 1996 Dutch Star?