There's no such thing as a "DS2" injection pump (as far as I know). Someone is getting numbers and letters confused. 6.2s and mechanically injected 6.5s use DB2 pumps. Civilian 6.5s from 1994 and up use electronic DS4 injection pumps. The new military 6.5s are still mechanically injected.
It took Stanadyne and GM many years to work the "failure" bugs out of the DS4 electronic system. Thus the bad rep. It finally became fairly reliable but expensive. I"ll take a mechanical pump anytime.
I do not believe that 6.5 got 14 MPG pulling that camper. I"ve had several 6.5s and over twenty 6.2s. Just pulling a 21 foot camper I tended to get around 13 MPG. My Ford 7.3s about the same and my 5.9 Cummins 2-4 MPG better.
I know of several Cummins 3.9s and 5.9s that got attached to GM 4L80E tranmissions. Took some work and I'm not sure it was worth the bother - but it certainly can be done.