In general doors have wood around the edges, maybe 3 or 4 inches. The inside is filled with foam, on the cheapest doors it is filled with air. There probably are one or two crosspieces, one at handle level for sure.
Now an RV door may be different. But in the above method you can trim an inch or two off the top and bottom and interior doors are often trimmed to fit. Not so exterior doors which is what you have.
How a pro would do itI still think you will be better off ordering a door in the right size.
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