About those bolts. They attach the body to the frame.
This is what it looks like from underneath:
Yours underneath probably won't look this rusty.
I don't think they salt the roads in Texas as much as they do up here.
Be sure it's just the bolt head and washer that are rusty and not the area around it also:
My van came with several deck holes from where the PO had installed shelving.
These holes came with their own rust issues.
Here is the method I used.
Grind off the rust at the top of the hole:
Underneath the van scrape the hole with a wire brush.
Treat both sides with a rust converter.
I used Purple Power deruster from Advance Auto.
Don't get it on your concrete.
After the converter dries cover the bottom of the hole with duct tape:
Cover the hole from above with the all powerful BONDO:
After the Bondo sets, sand and paint.
Since this will be hidden by decking I used off-the-shelf white auto spray paint.
Not worried about color matching:
Underneath the van, remove the duct tape:
Paint over the area with Rust Bullet: