I just sold a Cherokee that had a floor board in it, pretty much like your pictures.. I cut out as much rust as I could, and ground down all the surface rust, tack welded in some 16 guage sheet to fill in the holes and then sealed it with some seam sealer, the good seam sealer. If you can get that floor sealed up to where it can stay dry on the inside, it will last a long long time. As long as the holes are there and the moisture is coming in.. sitting under your carpet and stinking up your rig... it will continue to rot away till there ain't nothing worth repairing. You just need to get it dry and keep it dry under your carpet or floor liner and that will slow that rust down considerably.