The screen door is part of the entire entry door assembly. I replaced the door on my 1989 17-foot Bigfoot
with an off-the-shelf AP Products door.
AP Products 015-217711 Square Entry Door RH Polar White 24" x 68"
24 X 68 SQUARE ENTRANCE DOOR - RH WHITE [015-217711] - $635.73 : AP Products, RV/Mobile Home Hardware
I doubt if a new screen door will match up with an old door so replacing the entire door may be the only option. I could not find one in colonial white so I refinished it before installation but it was a direct replacement for the original door which had extensive dry rot within the 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.