I have a 2007 Winnebago Voyage 35L on a W22 Workhorse chassis with the 8.1 and Allison 6 speed transmission. So far I have found this to be a wonderful combination. Plenty of power for our needs and a smooth shifting transmision.
I can't tell you exactly when the last Workhorse was made, but it was a sad day when the production ended. Parts are still readily available for the engine and transmission. I've not had a need to look for any other chassis parts, but believe they will not be too hard to find.
In what is strictly my personal opinion, the Workhorse was one of the best gas motorhome chassis on the market.
Tom & Barb
'07 Winnebago Voyage 35L
Workhorse W22 Chassis