I bought my '81 last year and the day after I got it home I stepped inside the door and fell through the floor at the threshold.
I'm guessing if your floor is soft and rotten big time
at all corners, and, especially considering the very advanced age of your Burro
, your smartest move
is to remove the entire floor and replace completely with new. Big job, but could be well worth the effort since the floor is the only thing prone to rot. And rot will always come back to bite you, again and again. If the rot is not major maybe for you a chemical solution is the way to go. Check this site out:
I have not replaced an entire floor myself, but others on this site have, or have had it done for them. I have replaced about 2/3rds of rotted floor in my Burro
, piecing it in after making cardboard templates. Some times I think back on it and wish I had bit the bullet and did the whole floor. But, no nerve, or the time, or the place to do it.