I have a 17 ft Boler
trailer and my floor also gives when I walk on it, and dips at the door. An idea I had and found can be done ( talked to a trailer repair place here in Vancouver) is to weld angle iron to trailer frame and slip plywood pieces
in (like a waste tray holder under your barbecue) in order to reinforce the plywood floor. Have to be careful about welding heat and the wood/fibreglass floor.
Since you have gutted the trailer, you may have easier options. I know there is a detailed video on separating Boler trailer from trailer frame to replace floor and it is very good.