I have heard a couple of guys on here talking about repairs under the floor boards, but I know it is quite involved. There are some major craftsmen on this site, and so they are able to tackle projects that others may not consider. We have done quite a few upgrades on our camper,
And honestly my first try would be to try and gain access in around the ductwork opening, and try and shove some bracing in under there and then screw some decking screws into it. Either that, or a new layer of subfloor screwed overtop, and then a new piece of flooring.
The bad news here is that when we buy these trailers, sometimes the price calls to us louder than the technical details. Most sagging is caused by not enough support when the trailer was built.
When we bought our first trailer we did not give a second thought to the structure under the floor... Now we are about to buy our second trailer and we are looking at models that offer a one piece ibeam, and 5/8 plywood floor... Experience is a good teacher... But in the meantime, I am sure you can improve the floor you have. Just be aware that if you install a new subfloor... There are wires running underneath so it is good to have an electrical and pluming schematic to avoid further damage. I learned the hard way after piercing the shell of our trailer while installing a shelf with too long screws.
2003 21 ft Forrest River Shamrock