What is the floor in? I see you're in Florida. My son-in-law bought a fiver there after that big hurricane years ago. The bottom soon collapsed. Sponginess is usually caused by water infiltration. I'm sure if you give more information you'll get help. Is it a trailer or motorhome? What kind? How long have you had it? When did it become soft? What is its history?
'11 Eagle 320RLDS
'02 Ford F350 dually, unchipped 7.3 PSD, PacBrake,
6.0 cooler, Ford AIS and Isspro trans. gauge
Replaced old AirLift airbags