Adding to this extremely helpful post after doing this project following a windy camping trip where the side door was flying open.
The Dorman product for Chevy Express vans is too short, and costs over $50. Chevrolet dealer was no help. I didn't try a RV repair or Roadtrek.
The previous poster's link to piano wire springs is now "not available" on Amazon.
I found a good replacement from McMaster Carr. Item #9271k704 (it's actually made by Jones Spring Co; the Jones part number is TMR-0560-063). Very cheap - $5.35 for a six pack excluding shipping. Link to McMaster page for torsion piano wire springs:
https://www.mcmaster.com/piano-wire-torsion-springs/ AND they arrived in one day! The wire gauge was very, very close to our original broken spring...although who knows if it's truly original equipment after 20 years.
This is a project that may make you tear your hair out but I successfully removed one layer of the coils (to make it 4 like the original spring) and bent the spring with two sets of pliers working against each other until it worked. After trying a 180 degree overall angle it wouldn't reliably close the door catch as needed so I had to bend a tighter angle...approximately 120 degrees.
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