If the belly pan looks intact the insulation is more than likely all there in the floor. The trailers are as well insulated as they could make them, there is fiberglass insulation between the inner and outer skins. The main region of heat loss is transmission thru the ribs. We kept our 74 heated for a month in Wisconsin and you could see the rib outline in the snow that had fallen on the roof and curve to the sides.
Door gaskets, window gaskets, and insulating the roof vent openings will offer the most improvement in an older unit. Changing out the glass would be expensive and if a window frame were to be broken where do you get one or how many $$ does it cost.
One option is to have a professional security film company come and do the windows. This will protect them from shattering because the film will hold them together and may even cut down on heat loss. I am sure that with a bit of tint it will make it cooler in the summer.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato