Hi, Otis,
Besides that show, that account is in Norman Franks' and Alan Bennett's
The Red Baron's Last Flight: A Mystery Investigated, Grub Street, 1997, p.61. It may be elsewhere but that's where I first saw it. The son of Gunner Ernest Twycross of the Royal Garrison Artillery said his father reached the plane first and MvR
"gurgled or gasped: '...kaput,' and died. He said the words sounded like 'War es kaput,' but with the noise around he couldn't be sure, but 'kaput' came into it." I've seen this debated here and there, and I think Mr. Franks pops in here now and then so perhaps he'll comment.
I will add that IMO you should grab the book! Bursting with 21 April 1918 info.