From my days with boats (even more of a money pit than RVs!) I learned of a product called Boatlife GIT-Rot. I recently used it on my motorhome to restore the rotted plywood panel sandwiched in the side-wall behind the right hand-side mirror. I had to replace a damaged mirror that some dolt, who shall remain nameless, had managed to knock off going through a toll booth lane! :o The underlying plywood had rotted to the point that it would no longer hold the mounting screws for the new mirror. I thought that an expensive and difficult wall surgery was going to be necessary until I remembered the GIT-Rot. A half-hour's work and an overnight cure left the wall stronger than ever.
It's a thin, two-part liquid epoxy that you squirt into drilled holes in the rotted wood without removing the rotted portions. By capillary action it permeates all the spongy, rotted wood fibers and cures overnight, leaving a wood/epoxy amalgam that stronger than the original, undamaged wood. You should dry the rotted area as much as you can with a hair dryer or heating gun set on low, but the stuff will even work on slightly damp wood, as it did with mine.
Here's the YouTube video:
GIT-Rot demoI got mine at Amazon, here.Just make sure you follow the directions carefully when it comes to temperature, mixing, and curing time. Also, if you are drilling on the vertical side of your beam, slant the drilled holes downward to allow the epoxy to flow into the wood and not spill down the surface.
Good luck. I hope it works as well for you as it did for me.
