I had the same issue on my 1989
Bigfoot 17. The door was full of dry rot and was falling apart from the inside out.
I replaced the door with an off-the-shelf AP Products door.
AP Products 015-217711 Square Entry Door RH Polar White 24" x 68"
I could not find one in colonial white so I refinished it before installation but it was a direct replacement for the original door.
Once painted it only took a few hours one afternoon to remove the old door and replace it with the new one, which included a new screen door.