Andy has great insight - the itchless style bagged insulation is great stuff, and the bag itself is perforated but not nearly enough to stand humidity and moisture it was never meant to see when correctly installed inside someones house. Remember towing the trailer in rainstorms and hitting standing water, windows - door -vents left open in wind storm downbursts, leaks. Once fiberglass gets wet it only takes two forevers to dry out, three if its in the tube rolls
Our trailers had 2" bat insulation laid over the frame and crushed down in place by the flooring. Not really duplicatable, and I wish I could rid the entire frame of that glass wick, I had floor damage six feet from the rear compartment where the crushed glass drew moisture along the rusting ladder frame rail.
What the fiberglass is especially good at is dampening sound - but after dealing with Norway Rat & Mouse nests under the floor I swore it off, out, away, gone, repeatedly. The Reflectix type bubble foil is transparent to sound, a similar product
Prodex Products : Prodex Total Insulation : Insulation Wrap : Bubble Wrap Insulation : Insulation4less.com does dampen sound somewhat so I got the big roll.
Living in Minnesota I'm more concerned keeping heat in, the foil bounces heat back up almost three times better than bouncing heat down like air-conditioning the interior in Florida would need but it's still pretty good at R6 single layer.
I used foil-foam-foil Prodex with a 1/4 to 1/2 inch air gap - painted flooring for odor block and hygiene, put in single thickness Prodex 2-inch border strips at edges of frame cavities, glued & sealed the strips on both sides with Vulkem and fastened the cover layer of Prodex with monel rustproof staples, to get a larger airspace I set a small island of two layers of prodex in the center, attached the same. Sealing with polyurethane caulk made it a super neat job. I can also see using sheet foam, the fanfold 1/2" thick sheathing type, as another layer below the foil. That style foam is scissors cutable and easily set in...