Hi badib-yazdi,
I have no personal experience with a project of the type that you are undertaking. All I can do is offer links to what I have read and my ideas of what I would do if/when I have to do a floor replacement.
Since you have no belly pan, I would consider using a Marine grade plywood and epoxy coating it. Please see post #7 by RareStream in this thread about the coatings and how to apply them:
http://www.airforums.com/forums/f461...xy-137415.html
But like I stated above...this is just what I would consider doing if it was my trailer. By no means am I claiming that this is the only way to go, or necessarily the best, just how I personally would do it. There are other opinions/options out there. I would encourage you to search the site, do a little more reading.
There are members here with a much larger knowledge base than mine, and are obviously more experienced doing this type of job. Be patient, search and read the forums before you dive in.
Hope this helps.
Blair