Hey Casey,
The original door was basically a wood frame, filled with styrofoam in the middle, and sandwiched by two pieces of fiberglass reinforced plastic (FRP) panel glued to it all. Then there was aluminum trim all around the edge.
If you have all of the aluminum trim, I think it's easiest to build a new one, especially if you have some construction confidence. If your FRP panel is still good, you can reuse it, but new is available at Home Depot in a 4x8 sheet. Many times water will sneak down between the bubbles of the FRP panel and the aluminum trim on the bottom of the door (it was never sealed from the factory), making the wood wet, and causing it to rot. It's only a matter of time before the whole door bottom is rotted away and no longer structurally sound, aka ready to fall off.
If you can find a replacement door, it will likely be very expensive and special order. Most RVs produced in the last 20 years have radius top corner doors, so a surplus door won't fit.
Do you still have your four aluminum edge trim pieces? Lay them out on a table and take a measurement from inside the channel.