I have patched the big hole in the floor where the water tank drain was!
I bought a MIG welder (seems like a handy thing to have if you own a 50 year old all-steel motorhome) from Craigslist. It was a pretty sweet deal including the cart and a gas bottle, and a very slightly used Lincloln 140 welder.
I worked to get the steel patch as close of a match as I could to the hole, it had some pretty good gaps part of the way around, but was pretty tight in some places.
I welded carefully - not running a bead too far at a stretch so that I would not overheat the thin steel and cause it to sag. In some parts of the circle I was filling a gap of perhaps 3/8 ".
As you can see above, it is pretty lumpy and bumpy at this point, so I got my power grinder out and spent the next hour grinding away metal that it seemed to take mere minutes to put in place - 15 minutes of welding and an hour of grinding!
At this point, the deck is smooth enough to install the wood over, so I am done grinding. I painted with some primer to match the rest of the floor. I also sprayed underneath with some rust-oleum undercoat paint.
I am pretty happy with this patch - it is my first experience welding. I have signed up for welding classes at the local community college, I would really like to KNOW what I am doing instead of guessing.
More to follow!