The Allison 1000MH used in the W-series Workhorse chassis uses a different Autopark, the J72 system, and it was a reliable system. The Workhorse P-series chassis with GM Hydramatic tranny used the J71 Autopark system up through 2004 and it had its drawbacks, but that's a different animal.
I think you have some incorrect information. The 2003 Workhorse W20/W22 did NOT have an Allison 6-speed (it was a 5-speed). The 6-speed version of the Allsion 1000MH and 2100MH did not come out until 2006.
Ed Gray will no doubt have better and more complete answers. Meanwhile, you might want to look at the Workhorse Chassis Gide for info:
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL