Workhorse was formed in 1998 to take over the medium-duty chassis business from GM when the terms of a financial bailout required GM to abandon that market (along with shedding some other business lines). This company is not the same one that put its brand on garden tractors. It made bare chassis for products like step vans and motorhomes, initially building the P-chassis it acquired from GM and its own designs for heavier gross weights (W22, W24), then lighter W-series to replace P-series, and towards the end, a rear engine model.
Workhorse was acquired by Navistar in 2005, a rescue without a bankruptcy. Navistar dropped some of the lines that were not selling well (like the rear engine), added a heavier W-series for the top of the RV market, and did a lot of development on the commerical lines, using smaller more efficient engines, including some of their own.
When GM stopped supplying the Vortec 8100 engine in 2007, Workhorse got out of the motorhome chassis business, concentrated on the step van market. This meant NaviStar owned two companies competing in the same business, so they eventually sold the Workhorse brand to electric vehicle manufacturer AMP. AMP is supporting the legacy W42 and W62 lines, but their primary business is electric trucks for the urban delivery market.
There are several independent businesses that specialize in providing parts and service for Workhorse motorhome chassis (P-series, W-16-18-22-24 et al.