You may be sorry insulating and sealing that floor unless you will store the trailer in a garage. If stored outside, that insulation will draw moisture and vermin will be living in the insulation in a couple of years. Not much heat is lost through the floor so that much insulation is uneccessary.
It seems like everyone here overbuilds their trailers to the extreme. Why does it need ot be so heavily built? This is a 1.5 ton trailer to carry 1000 lbs of stuff max and the frame could support 3 tons.