When we removed bolts from our sofa from our 2008 Tiffin Allegro Bus (we put in recliners), we found the bolts were just put in all the way through the slide floor. We just knocked the carriage bolts out and filled the holes with caulk from the top and the bottom and never had a problem with it for 75,000 miles. When Tiffin builds their slideouts, the floors are covered with Aluminum before they are installed so their bolts go all the way through when they install things like sofa's and seat belt fittings. A bit different from the way Fleetwood seems to build theirs
I'd use the old bolts if you can, and if not just drill all the way through the floor and under-cover for any new bolts you need. Use carriage bolts to keep the heads from catching on anything and caulk them well from the bottom and you should be fine.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad