GM has been in negotiations about replacing the Express van for about the last 5 years, but in a search not long ago, I couldn't find any final answer. I do know that Roadtrek is have a difficult time getting platforms for conversion.
So... your guess is as good as anyone elses. I don't think GM even knows for sure yet.
Mumkin
2021 Promaster 1500 188wb conversion
2019 Roadtrek Simplicity SRT (half Zion/half Simplicity)
2015 Roadtrek 170
2011 LTV Libero
2004 GWV Classic Supreme