Floor rot

The floor rot under the refrigerator near the entry door is the worst spot in this trailer. I cut out the plywood. leaving a hole about 18 inches or so across. Not sure (since I don't weld.) how to fix this and make it strong. I am thinking a 2X10 piece of wood with l brackets for extra support and some JB Weld on those as well. might just strengthen the area, then I can screw the new plywood into to outriggers also.
If that works I can then seal up the seams with some Bondo. Really I should pull off the"banana wrap" and loosen the belly pan there, to weld in a new outrigger and metal along the outside edge. But I don't weld.
SO I have to think outside the box on this one.
Luckily for me this is the only truly screwed up floor spot I found other than one under the tub where the roof vent had leaked..for a long time. That one was easier to fix and now is sealed up tightly.  
I also used some wood hardener, and Bondo as needed around the edges in the main room, where they may ever have been a leak, or might be some time in the future. The living room floor is solid.