If you are crawling under your camper and looking up, and you see plywood above the frame (the floor), then you do not have any insulation of any kind under your camper. There should, at a bare minimum, be a cover inclosing the underbelly of your camper. That enclosure should be on the bottom side of the frame, the frame in the middle, and then the floor of your camper.
The older style of underbelly was a Dracor style fabric, (like a tarp) that had insulation adhered to one side and the other side (the exposed side to the road below) was a tarp type material.
The other type of covering (that sounds like it's missing) is called Chloroplast. It's the same material that is used in making yard signs for, House for Sale, For Sale by Owner, Yard Sale, Private Property, and almost every political candidate running for office sign people put up in their yards. It does not have insulation affixed to it, but there is a layer of insulation that lays on top of it.
If neither of these are there, then probably the previous owner had some kind of issue that required him/her to get in that space between the frames for some kind of a repair (most likely plumbing). Maybe they removed the heat ducks that originally ran under the floor to keep that space heated. And realized they did not need to replace the insulation or the under belly covering, or it was simply too much work to replace it.
Either way, it sounds like yours is gone. Depending upon how you plan on using your camper, it does not hurt anything to leave that space open as long as you winterize your pipes before things freeze up.
If you plan on using the camper in freeze zone areas, then you'll either need to (1) Winterize the pipes and use the camper "dry"... no water at all until you get past the freeze zone months, exaxtly as you would as if you are tent camping. No water down the drains, no water in the tanks. or (2), replace the underbelly, change the furnace vents so air blows under the floor again, and insulate. or (3) if the trailer does not move or is permanently set in one spot .... put us some heavy duty skirting and run heat under the trailer.
To plant a garden is to believe in tomorrow.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually