The most effective way I know of is a lot of work...removing the interior panels and placing new batts between the skins. Same thing with the floor, but you take off the belly pan. If your floor is "bad" you can fix this at the same time...
There have been some discussions here about using spray-in-foam-type insulations, you may want to perform a search through old threads using "insulation" as a keyword.
Shari
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