When should you replace? I am not a carpenter, so just take my personal opinion from experience fixing one camper floor section. Any structural wood that has lost maybe more than 20 or 25% of its useful cross section should probably better be replaced than repaired? Remove as much as needed to find solid wood and a good place to anchor to, then splice in a new piece.
Use an oscillation saw such as a Fein cutter or one of the cheaper knockoffs if you don't have one yet. It will make it a lot easier to make awkward, difficult cuts without tearing everything apart.
Wood that is not really structural, like camper wings or luan/plywood sheets covering something, I would probably not hesitate to fix with any of the methods mentioned above. Although these parts are often just as easy to replace than to repair...
