repair floor / walls?

If you've got to ask how to do it, it is plain that you are not qualified to do it. No disrespect intended.
I'd take the nut off the sunken screw, and if the wood is not too rotten, get a plate of metal and use this as a large washer, to spread the load. Making sure I'd treated the wood for any damage.
But if I were like you, and not too sure of myself, I'd take it to a repairer. especially if you intend to drag it down some motorway.
IF YOU INTEND TO TOW, GET IT CHECKED OUT!!

CHUMLEY
January 2010