I had the same problem on my '95 Itasca Suncruiser. This seems to be a common problem on '90's Winnebago products.
The soft floor is a result of delaminated "sandwich" floor material, which is 3/4" styrofoam glued between two layers of plywood. The root cause is inadequate supports underneath the floor.
There are two basic repair methods. The preferred method is to install better supports, you can do this in many ways. I ran two 13' pieces of 1 1/4" square tubing inside of the two existing support rails, which were more than 3 feet apart. Now my flooring only spans a little more than 1 foot and is solid.
The other method used for areas that cannot be accessed from basement storage areas is to remove the floor covering, cut out a piece of the top layer of plywood, cut out a smaller piece of the styrofoam, then glue plywood pieces back in the holes using a construction grade contact adhesive.
I called Winnebago Customer Service and they faxed me their floor repair instructions which go into more detail than I did above and include some drawings.
Dale & Becky
'95 Itasca Suncruiser 37'