Remember, Workhorse is NOT a GM product. While it does use the GM engine, Workhorse was a separate company that built RV Class A chassis starting in 1999. They took the P30 chassis and upgraded it and the drivetrain.
While it was supposed to have gone out of business in 2009 due to the economic downturn, it seems that it recovered by 2015 where it planned to re-enter the chassis market once again. They did not
Even though this has taken place, it may still be hard to find parts. Some Chevy Workhorse chassis owners do have difficulty in locating the right parts for their older RVs that have the Chevy Workhorse chassis in them.
Since Ford is still producing(and has parts), I would state that Ford is a better choice for a 20 year old motorhome. Also, does the Workhorse have the electric driven Ignition interlock which causes problems when it overheats or breaks? If so, this alone would disqualify me from having one. Doug