Possible New HTT - 21sb

Just replaced some of the floor on my 06 18sb. While I agree that the crossmembers are too far apart at 3' I could not feel any deflection until the floor started to rot. The membrane underneath is a Tyvek type material, it was/is glued to the bottom layer of 3/th's plywood, it should feel solid and taught when pressed from below. If there are any loose areas then the lower layer of plywood was turned to mush like mine did. Take along a dolly and check the bathroom area as well as near the door, water heater, sink, and the whole front. I had no indications of a problem from the inside other then one soft spot just in front of the sink but the front 4' was shot. The water gets trapped on top of the Tyvek, hence the bottom wood goes first.
The softness on mine stemmed from the lower plywood falling apart which weakened the Styrofoam sandwich which eventually broke midway between 2 of the cross members,