My son and I just replaced the entire floor in a bedroom slideout. Not easy, but do-able with basic carpentry skills and tools. Some of the working positions are a PITA, but just keep at it a little at a time.
An oscillating tool with flush cut blades is a gift from the Gods! Reach in under frames and cut out bad wood. We reframed with 1x6 PT (look for the enviro friendly brands. No VOCs) then used 3/4" foil faced insulation board and a Luan subfloor. (Remember we were in the closet.) You could use Marine Ply.
Good Luck!