How big is the spot? A better definition of soft? Does it just give when walked on? This is difficult understanding it completely. Many times when we were in the market for TT & fivers there were "new" models at the RV shows we attended that had soft "areas" as we walked through them. Different mfg and models. I made up my mind then we would never buy one where the floor gave.
Some are made cheaper with the floor decking not that thick to begin with or well supported.
If it's over the storage area perhaps you can get look in there and see if you see any rotting or bracing issues.