For the shell on floor insulation...
Cut the rigid foam insulation to approximately fit each of the cavities in the framing.
Then cut the leftover scraps into about 1.5" strips, put double sided tape onto both faces of the strips
Tape the scrap bits roughly 1 inch in from the perimeter to the underside of the floors, tape some more strips in the middle of bigger cavities. The tape is not structural it is just to hold it in place well you work.
Once the strips are in place peel the backing off the double sided tape and position the large panels that you cut first into position
Then drill stainless steel screws with an added washer that are long enough to get through the two combined pieces of foam board and in to the sub floor about half an inch. I say stainless because I can tell you the factory not stainless ones that were used in the perimeter of my trailer all rotted away
Now comes the messy sticky part, Buy a couple cans of CLOSED CELL spray foam insulation, closed cell does not absorb water.... open cell is like a sponge. Place a tarp under the work area, wear protective eye wear and a tight fitting hat or a shower cap and spray into all the perimeter gaps. Some of the spray foam will fall down, work at an angle and don't let it get you. I dragged my ponytail through some that hit the tarp, it was part of me for months... and the plus side though, after it dries it pulls right off the tarp, not so much for hair...
On my trailer I did not spray the outer edge until just as I was wrapping the sides over the belly pan area.
Be advised that the foam board really stinks, I brought mine home in my car and it made my eyes water, I hosed it off and left it outside a few days before using then I left the belly pan open a month before wrapping her back up. It just gives it more time to off gas before trapping the smell in your trailer.
This method gives a closed air gap that will keep your trailer a wee bit warmer on the feet and you happier in the long run.