I would go in from underneath if possible. Remove or cut an access in the underbelly and see what is going on. Easy to put in a brace under the soft spot unless it is water damage then you will want to fix that first.
2014 Jayco Swift 281BHS
2015 F250 XLT 4x4 Crew Cab, Short box, 6.2 gas
Solar powered! The trailer not the truck!